Bets convert the finest (4)
I believe the answer is:
best
'the finest' is the definition.
(I've seen this in another clue)
'bets convert' is the wordplay.
'convert' indicates an anagram (I've seen 'converting' mean this).
'bets' with letters rearranged gives 'BEST'.
(Other definitions for best that I've seen before include "Beat" , "Most excellent - late football star" , "at 16 down [MAN UNITED] ?" , "Of the highest quality" , "Top, finest" .)