Across:
1 Invent (DEVISE) a top missing from videotapes (‘a top’ missing from ‘videotapes’ = videes = DEVISE),
4 Upsets (= reversal indicator) English (-E) Fellow (man = NAM-) and most of the rest (most of the ‘rest’ = res = -SER) at university (U-) (USERNAME) with online identity (USERNAME),
9 Blow the whistle on someone locally (blow the whistle on someone locally/inform = SING-) in hospital (-ER) (SINGER) working in theatre (SINGER),
10 Maybe get (maybe get) lost (= anagram indicator) (maybe get = MEGABYTE) in storage facility (MEGABYTE),
12 Men-only party in The Palace perhaps (ROYAL STAG) is the icing on the cake (ROYAL icing) for one of those in The Park (STAG) (ROYAL STAG),
13 Locally, treat roughly (ROUST) my group (-US-) found in The Hill (tor) going all the way back (= reverse indicator) (tor = RO-T) (ROUST),
14 Keeping an eye on the eagles and albatrosses (BIRDWATCHING) for peer (-WATCH-) following (= position indicator) high-flyer (BIRD-) hot (-IN-) at golf (-G) (BIRDWATCHING),
18 7 down (‘asylum’ = ASYLUM) from The Hunter (SEEKER) (ASYLUM SEEKER) has left home and wants to live in Ireland, for example (ASYLUM SEEKER),
21 School (STYLE) report (= homophone indicator) found over the wall (‘stile’ = ‘STYLE’),
22 Church leaders (HIERARCHY) greet (HI-) rare (rare) review (= reversal indicator) (rare = -ERAR-) that’s a fair bit (half of) touchy (half of ‘touCHY’ = -CHY) (HIERARCHY),
24 Bolt (U- bolt) for safety mechanism (safety pin = -P IN) on guns (ARMS) (UP IN ARMS) when at boiling point? (UP IN ARMS),
25 Log in leaving invigilator (‘log in’ leaving ‘invigilator’ = viiatr = TRIVIA) the unimportant facts (TRIVIA),
26 One of The Greens (SIDE DISH) in Dublin (SIDE) on course (DISH) (SIDE DISH),
27 Wait (S-ERVE) around The Point (compass point west = -W-) (SWERVE) to change direction suddenly (SWERVE).
Down:
1 Outline (DESCRIBE) of first dictionary (first ‘Dictionary’ = D-) by English (-E-) writer (-SCRIBE) (DESCRIBE),
2 Victory (V-) in (-IN-) last race (last ‘racE’ = -E-) for stable? (-YARD) (VINEYARD) Grounds for producing bottles of champagne! (VINEYARD),
3 Poach (STEAL) starter of scallops (starter of ‘Scallops’ = S-) and duck (-TEAL) (STEAL),
5 Transports toys (SLEDGE-) by Beetle (beetle -HAMMER) (SLEDGEHAMMER) and is employed in hard labour? Presumably that comes as a blow! (SLEDGEHAMMER),
6 Orchestrate New Order (REARRANGE) back (REAR-) catalogue (-RANGE) (REARRANGE),
7 Unanimously dismisses union (‘unanimously’ dismisses/drops ‘union’ = amusly = ASYLUM) for those seeking shelter (ASYLUM),
8 Sets up (ERECTS) secret (secret) organisation (= anagram indicator) (secret = ERECTS),
11 Majestic (STATELY) city in Syria (Homs = HOM-S) overlooking Euphrates, primarily (‘Euphrates’ primarily = -E-) (STATELY HOMES) built for the aristocracy to live in (STATELY HOMES),
15 Desert (WASTELAND) shady type (pastel) losing the head (p) (‘pastel’ without ‘p’ = -ASTEL-) in The Magic Bar (the magic bar = wand = W-AND) (WASTELAND),
16 Southern (S-) Kerry (-KY) report (COVER) (SKY COVER) found in The Cloud (SKY COVER),
17 Private space (ARMY BASE) for major employer (ARMY BASE),
19 Broadcasts (ISSUES) to lots of children (ISSUES),
20 Countless (MYRIAD) millions (M-) to back (= reversal indicator) milk producer (dairy = -YRIAD) (MYRIAD),
23 Pointer (ARROW) to a (A-) royal (-R-) line (-ROW) (ARROW).