In favour of appropriate gain (6)
I believe the answer is:
profit
'gain' is the definition.
(I know that gain can be written as profit)
'in favour of appropriate' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'appropriate' becomes 'fit' (similar in meaning).
'pro'+'fit'='PROFIT'
(Other definitions for profit that I've seen before include "Clean up" , "Net gain after expenses" , "Use" , "Advantage, benefit" , "It's gained on a transaction" .)