Only fair (4)
I believe the answer is:
I believe this clue is a double definition.
'only' is the first definition.
('just' can be a synonym of 'only')
'fair?' is the second definition.
('just' can be a synonym of 'fair')
(Other definitions for just that I've seen before include "By the skin of your teeth - equitable" , "Hardly" , "Fair-minded" , "Barely - deserved" , "Fair - precisely" .)