Hardly fair (4)
I believe the answer is:
just
I believe this is a double definition.
'hardly' 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 "This very minute" , "Ethically right" , "And nothing more" , "By the skin of your teeth - equitable" , "legal?" .)