Perfectly fair (4)
I believe the answer is:
just
I believe this clue is a double definition.
'perfectly' is the first definition.
(thesaurus)
'fair' is the second definition.
('just' can be a synonym of 'fair')
(Other definitions for just that I've seen before include "Barely - deserved" , "Fair; a moment ago" , "Merited" , "Morally right" , "Fair-minded" .)