This is only fair (4)
I believe the answer is:
just
I believe this clue is a double definition.
'this is only' is the first definition.
(I've seen this in another clue)
'fair' is the second definition.
(synonyms)
(Other definitions for just that I've seen before include "Merely right?" , "Merited" , "By a whisker" , "Quite" , "Morally right" .)