Unhappier (6)
I believe the answer is:
sadder
'unhappier' is the definition.
('sad' can be similar in meaning to 'unhappy')
This is all the clue.
(Other definitions for sadder that I've seen before include "Unhappier" , "extra-blue?" , "More unhappy (but hopefully 1 down)" , "Gloomier" , "More downcast" .)