Top bets are lost (4)
I believe the answer is:
best
'top' is the definition.
(similar in meaning)
'bets are lost' is the wordplay.
'are lost' indicates an anagram.
'bets' anagrammed gives 'BEST'.
(Other definitions for best that I've seen before include "Cream of the crop" , "Top, finest" , "Most advantageous" , "Beat" , "Superlative" .)