Wager about small top (4)
I believe the answer is:
best
'top' is the definition.
(similar in meaning)
'wager about small' is the wordplay.
'wager' becomes 'bet' (synonyms).
'about' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bet' placed around 's' is 'BEST'.
(Other definitions for best that I've seen before include "Cream of the crop" , "Supreme" , "Most suitably" , "12 21[FOOTBALLER" , "Worst -- as good as can be" .)