Greatest bets are lost (4)
I believe the answer is:
best
'greatest' is the definition.
('best' can be similar in meaning to 'greatest')
'bets are lost' is the wordplay.
'are lost' indicates anagramming the letters.
'bets' is an anagram of 'BEST'.
(Other definitions for best that I've seen before include "The finest - worst" , "Optimal" , "at 16 down [MAN UNITED] ?" , "Of the highest quality" , "Supreme" .)