Quintets solution
by Kegler

As indicated by 23 Across, MARCH MADNESS, seven clues incorporate an anagram of the letters M-A-R-C-H.

Quintets solution grid

Across
1PAP + I.E. + [RMACH] + E
10BUSS ("BUS" hom)
11NOW + HER + E
13ENS(NAR)ED (RAN rev; NEEDS anag)
14N(ESTLING),G (SILENT anag)
16C + O + B
18COP + Y
20ALBUMen
22V(AN)ILLA
23MARCH MADNESS
26ALCHEMY (LECH MAY anag)
29ELIZA (hidden)
30ANTE (NEAT anag)
31K + I'M
32A[RMCHA]IR
35F(REAK)ISH (RAKE anag)
37R(ENT)ING (NET anag)
38LATE (TEAL anag)
39STA[RCHAM]B + E,R
Down
2AU + REO + LA (ORE anag)
3P.S. + IS
4ISN'T (SNIT anag)
5EYELASH (rev hidden)
6MOS (1st letters)
7[CHARM] + IN + G
8HERCULEAN (CLUE HE RAN anag)
9WE'D
10BE + N + [CHMAR] + K
12REOIL (ROLE I anag)
15NBA (BAN anag)
17BRASS + ERIE
19P[ARCHM]ENT
21SCHEMATA (hidden)
24D[RACHM]A
25SAT + I + ATE
27LO(I)RE
28MIR (RIM rev)
33HE(L)M
34ARAB (hidden)
35FRY (2 defs)
36IN + C