Not just (6)
'not just' is the definition.
('unfair' is the opposite to 'just')
This is all the clue.
(Other definitions for unfair that I've seen before include "Not just - lacking in beauty" , "deceitful" , "Dirty" , "out of order" , "Weighted" .)
(Other definitions for invalid that I've seen before include "Not true" , "What Bunbury was, permanently" , "patient [found]" , "Nullified" , "One disabled by illness" .)