In favour of suitable financial gain (6)
I believe the answer is:
profit
'financial gain' is the definition.
(profit is a kind of financial gain)
'in favour of suitable' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'suitable' becomes 'fit' (similar in meaning).
'pro'+'fit'='PROFIT'
(Other definitions for profit that I've seen before include "Yield - gain" , "Gain from employment of capital" , "Use" , "Gain, return on investment" , "Surplus earnings" .)