One questioning what's in mailbag -- no stickers? (8)
I believe the answer is:
agnostic
'one questioning' is the definition.
(I've seen this before)
'in mailbag no stickers?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AGNOSTIC' is hidden in the letters of 'mailbag no stickers'.
'what's' is the link.
(Other definitions for agnostic that I've seen before include "One who is undecided" , "doubtful, I think" , "uncertain about what to believe" , "He doesn't believe" , "One believing God is unknowable" .)