Gambler improved (6)
I believe the answer is:
better
This clue is a double definition.
'gambler' is the first definition.
(better is a kind of gambler)
'improved' is the second definition.
('better' can be similar in meaning to 'improved')
(Other definitions for better that I've seen before include "Surpass; gambler" , "recovering from illness" , "Gambler; more effective" , "Improved - gambler" , "One making wager; more advantageous" .)