First Off solution
by Kegler

In nine cases, the first letter of an entry is “off” (shown below in red); in clue order, these unused letters spell SQUARE ONE.

First Off solution grid

Across
1S + EDUCING
6POSH (SHOP anag)
9ER(NO)EW (rev)
11QUIT + E
12SNEEZE (ZEN SEE anag)
13TEATIME (MITE ATE anag)
14BIRD (2 defs)
15UR GENT
16GALENA (ANGELA anag)
17FOR + ET + OP
22SURREAL (A RULER'S anag)
26PRO(EM)S (ME rev)
28MO(RNO)E (RON anag)
30REST (hidden)
31ENG. + RAFT
32ACORNS (CARSON anag)
33WETLY (odd letters)
34ENLIST (LISTEN anag)
35RE(A)D
36ETHY + LE + NE (THEY anag)
Down
1RENTAL (ANTLER anag)
2D(EMAG)OG (GAME rev)
3UNITER (TRIUNE anag)
4COSINES ("COSIGNS" hom)
5NONE + GO
6P(UEBL)O (BLUE anag)
7E + TE(RN)AL
8HEYDAY (rev hidden)
10RUdER
11SELA (ALES rev)
17F(ORES)EE
18OR + OG + E + NY (GO rev)
19LARAMIE (A REALM I anag)
20S + PRAW + L (WARP rev)
21RE + T + OLD
23USENET (hidden)
24ENROL + L (LONER anag)
25PET + IT + E
27NARY (YARN anag)
29jEFFS