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