In favour of helping balance (10)
I believe the answer is:
proportion
'balance' is the definition.
(synonyms)
'in favour of helping' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'helping' becomes 'portion' (synonyms).
'pro'+'portion'='PROPORTION'
(Other definitions for proportion that I've seen before include "Part - one needs a sense of it" , "balance" .)