Gain made by professional in good condition (6)
I believe the answer is:
profit
'gain' is the definition.
(I know that profit is a kind of gain)
'by professional in good condition' is the wordplay.
'by' is a charade indicator (letters next to each other).
'professional' becomes 'pro' (abbreviation).
'in good condition' becomes 'fit' (I've seen this in another clue).
'pro' next to 'fit' is 'PROFIT'.
'made' acts as a link.
(Other definitions for profit that I've seen before include "financial advantage" , "Gain, return on investment" , "Financial gain" , "returns" , "Net gain after expenses" .)