Wagers are best lost (4)
I believe the answer is:
bets
'wagers' is the definition.
('bet' can be a synonym of 'wager')
'best lost' is the wordplay.
'lost' is an anagram indicator.
'best' with letters rearranged gives 'BETS'.
'are' acts as a link.
(Other definitions for bets that I've seen before include "Wagers" , "Makes wager" , "Wager, stake" , "Gambles" , "plays" .)