In favour of healthy gain (6)
I believe the answer is:
profit
'gain' is the definition.
(I know that gain can be written as profit)
'in favour of healthy' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'healthy' becomes 'fit' (associated in meaning).
'pro'+'fit'='PROFIT'
(Other definitions for profit that I've seen before include "Use" , "Gain from employment of capital" , "Money made" , "It's gained on a transaction" , "Clean up" .)