Away, drinking can, not overindulging (9)
I believe the answer is:
abstinent
'not overindulging' is the definition.
(abstaining from certain things)
'away drinking can' is the wordplay.
'away' becomes 'absent' (similar in meaning).
'drinking' indicates putting letters inside.
'can' becomes 'tin' (synonyms).
'absent' enclosing 'tin' is 'ABSTINENT'.
(Other definitions for abstinent that I've seen before include "without booze" , "On the wagon" , "Refraining from drinking alcohol" , "Not indulging appetite for food or drink" , "Restrained" .)