Worst gamble accommodating son (4)
I believe the answer is:
best
'worst' is the definition.
(both can mean to beat someone)
'gamble accommodating son' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling**).
'accommodating' is an insertion indicator.
'son' becomes 's'.
'bet' going around 's' is 'BEST'.
(Other definitions for best that I've seen before include "Most excellent - late football star" , "The finest - worst" , "George -, footballer" , "Most suitably" , "BOSOM] ?" .)