Manifest Destiny solution
by Kegler

As indicated by 30 Across, GO WEST, YOUNG MAN, the six answers containing synonyms (well, near-synonyms, anyway) of “young man”
have that synonym “go west”, i.e., move as far to the left as possible within the entry.  These synonyms are shown in red in the grid and answers.

Manifest Destiny solution grid

Across
1SU(P)ERS+ ON ICe + ALLY [SON "goes west"]
10OVA (hidden)
12wOLLAS (rev)
13BrIDE
15FLA(MBO)Y + ANT (MOB anag) [BOY "goes west"]
16ET AL (rev)
17AFRaid + IT
20pATE
22RU(GRAm)T
23CADENCE ("KAY DENSE" hom)
25OVERSIMPLIFY (FOR EVIL SPY I'M anag) [IMP "goes west"]
28TAB + LO + IDS
30GO WEST, YOUNG MAN
34LING + U + IN + I
39CON + Q + UISTADOR (AUDITORS anag) [TAD "goes west"]
40G(O + ODD)AY
42AN + CHORe
43EARl
45SWEAR (hidden)
46CAPE (PACE anag)
47HAND (P)UPPET (END UP AT HP anag) [PUP "goes west"]
50A(R)ID
51TEARS (2 defs)
52USE (SUE anag)
53SHOULD + ER + B + LADiES [LAD "goes west"]
Down
1SO(BER + I)NG (I REB rev)
2N(A)Y
3S + AFFRONt
4P + LAITy
5ELM (hidden)
6ISN'T (SNIT anag)
7CITED ("SIGHTED" hom)
8LIT + ANY
9YELPERS (PRESLEY anag)
11VO(yLUj)ME (MOVE anag; jULy rev)
14DAN (C) + ED
18RA(V)E
19TARTY (RATTY anag)
21D(beEF)OG
24A + L + BUM
26PA(W)N
27UTICA (hidden)
29I(MU)S
31EG + ADS (SAD anag)
32O + INKS
33NARRATES (RARE ANTS anag)
34LO(GI)CAL
35I + SO(B)AR
36JUNE (1st letters)
37PICADOR ("PICK A DOOR" hom)
38B + OOZES
39TORPID (TRIPOD anag)
41D(EPT)H) (PET anag)
42A(WAR)D
44AUTO (rev hidden)
48HA + L
49P(U)B