Missing the Big Picture solution
by Kegler

The theme is the old adage "You can't see the forest for the trees."  30 Across, WHAT YOU CAN'T SEE, describes  
THE FOREST, the nine letters from the Across entries that get overwritten.  8 Down, WHY YOU CAN'T SEE IT,    
describes the "trees" from the Down entries that overwrite those letters. The colored squares in the answer grid indicate
the following replacements: 1 YEW for T, 2 PEAR for H, 3 FIG for E, 4 ASH for F, 5 ELM for O, 6 TEAK for R,    
7 OAK for E, 8 CEDAR for S, 9 PINE for T.

Missing the Big Picture solution grid

Across
1A + G(END)AS
8WOM + BAT (rev)
12BO(THE)R (ROB rev)
13O + OH
14V + AM + P,S
16RUED (RUDE anag)
17EIGHT ("ATE" hom)
18LATCHKEY ("CATCH LEE" spoonerism)
20A + F + RAID
22HOWLING (hidden)
23SE(RENE'S)T
26H(ENC)E
27ESCORT (SECTOR anag)
30WHAT YOU CAN'T SEE
33EACH (ACHE anag)
37NONE (hidden)
38HIRE (HEIR anag)
40T + ROLL
41HA(ZE)D
42C(ET + ACE)ANS
44SA + LA(R)Y
45DE(P)LETE
46MI + L + LION
47TI(T)LE
48E + ASE (SEA anag)
49LEI + SURE
50NUDE (DUNE anag)
51ME(L)TS
52MUSH (HUMS anag)
Down
1A + B(R)EAST
2GO + U + L{ASH}ES
3E + {YE(W}E)AR
4D(E)ICING
5ARCH (2 defs)
6SmOCKS
7DO + G + {FlIG}HT
8WHY YOU CAN'T SEE IT
9OVERWH{E + L + M} (HOWEVER anag)
10MAIL (2 defs)
11A + P{(PEA)R}ANCE
15STAG + E (GATS rev)
19PINT-SIZE (NETS I ZIP anag)
21DEMO (hidden)
24REAC + T (CARE anag)
25ENTHRALLED (HELD RENTAL anag)
28SCOUNDREL (CRUEL DONS anag)
29WEE + D
30WEL(COME)D
31UNLAY (rev hidden)
32T(HALL)I + UM
34ABE{CEDAR}IAN (CABINED AREA anag)
35CO + CA(I)NE
36CL{OAK}ROOM (LOOK, MARCO anag)
39{TEAK}ETTLES (LET KATE SET anag)
41H(A + P + {PIN)E}SS
43TEETH (hidden)