ACROSS
1 ADMIT (ad(m)it); C removed from clue [core]
5 BRIBING (b(rib)ing); T added to clue [sopping]
9 READS UP (pursued a); U added to clue
10 DELE (avant-garDE LEssons); O removed from clue [omit]
13 SLAT (slate B e); R added to clue [tail]
15 SEESAW (see + was); N added to clue [yaw]
16 ARSE (ears); P removed from clue [pitched]
17 THETA (threat); R added to clue
19 NSEC (since); I added to clue
21 MSG (2 defs.); G added to clue [is]
22 RSVP (Reckless Sexy Vixens Play); Y removed from clue [party]
24 CROESUS (choruses); H added to clue
26 ADELINE (A(d + E)line); G removed from clue [song's]
27 DOER (rode); T added to clue [busy]
29 TWO (too); R removed from clue [three]
31 DUET (due + t); T added to clue [sing]
34 BRITA (br(it)a); O added to clue [purer]
37 ARES (Mars B M + E); I removed from clue [chief]
38 REHANG (he rang); D removed from clue [wildly]
39 BYTE (bite); A removed from clue [hear]
40 DOLT (do(l)t); U added to clue [cloud]
41 EMBASSY (empty B P.T. + bass); S added to clue [low-speaking]
42 HYSSOPS (hy(essop)s); E added to clue
43 ORTHO (nORTH Oregon); N removed from clue [lawns]
Down
2 DEERE (dear); D removed from clue
3 MASSIVE (mass + I've); A added to clue [stout]
4 IDLES (slides); S added to clue
5 BUTT (two defs.); O added to clue [ram]
6 IDEE (I'd + see B s); T removed from clue [thought]
7 BEET-RED (be(etre)d); H removed from clue [flush]
8 NEARS (germaN ARSenal); E removed from clue
11 SWIGS (swings B n); N removed from clue [number]
12 FAIR (2 defs.); C removed from clue [clear]
14 AENEID (I need a); U removed from clue [tousled]
18 HERE (h + ere); V added to clue [ender]
20 COSTLY (Lycos); T removed from clue
23 PLURALS (p + l + Urals); O removed from clue [nouns]
24 CNUT (saraCeN UTterly); E added to clue
25 SOUREST (tosser); U removed from clue
26 ATARI (cheAT ARIzona); T removed from clue [Tempest]
28 RUST (trust B t); R added to clue [tip]
30 WEEDY (Dewey); E removed from clue [spare]
32 QATAR (cotter); L added to clue [peasant]
33 YEESH (see ya + h); A added to clue
35 INTO (tINy TOons); Y added to clue
36 ABMS (MBA); S removed from clue
The letters deleted from the clues, in order, spell "COPY GRID AND THEN CUT OUT E'S". The letters added to the clues, in order, spell "TURN RIGHT TO USE AS OVERLAY". Copying the grid, cutting out the E's, rotating the grid 90 degrees to the right, then laying it on top of the solved cryptic grid gives the message "ADD LETTERS IN SQUARE HOLES".
