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' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bet' enclosing '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" .)