Right argument in favour for each (6)
I believe the answer is:
proper
'right' is the definition.
(similar in meaning)
'argument in favour for each' is the wordplay.
'argument in favour' becomes 'pro' (as in pros and cons).
'for each' becomes 'per' (I've seen this in another clue).
'pro'+'per'='PROPER'
(Other definitions for proper that I've seen before include "Appropiate, right" , "Correct, decent" , "Correct; formal" , "Genuine; respectable" , "Suitable and correct" .)