Skinflint fit to be unhappy (9)
I believe the answer is:
miserable
'unhappy' is the definition.
('miserable' can be similar in meaning to 'unhappy')
'skinflint fit' is the wordplay.
'skinflint' becomes 'miser' (I've seen this before).
'fit' becomes 'able' ('able' can be similar in meaning to 'fit').
'miser'+'able'='MISERABLE'
'to be' acts as a link.
(Other definitions for miserable that I've seen before include "sorrowful" , "Depressed" , "Being sad" , "Blue" , "Glum, wretched" .)