Gambler not doing well with additional funds? (6,3)
I believe the answer is:
'with additional funds?' is the definition.
(better off can mean with more money)
'gambler not doing well' is the wordplay.
'gambler' becomes 'better' (someone who makes bets is a gambler).
'not doing well' becomes 'off' ('off' can mean feeling unwell).
(Other definitions for better off that I've seen before include "wealthy", "Richer", "in happier financial circumstances", "having lost less", "Someone setting out for the bookies".)