Risk only half improved (3)
I believe the answer is:
bet
'risk' is the definition.
(thesaurus)
'only half improved' is the wordplay.
'only half' suggests halving the letters.
'improved' becomes 'better' (similar in meaning).
'better' halved is 'BET'.
(Other definitions for bet that I've seen before include "'Stake, wager (3)'" , "Wager, back" , "Wager, speculate" , "Speculation" , "Wager, gamble" .)