Most difficult (7)
I believe the answer is:
hardest
'most difficult' is the definition.
('hard' can be a synonym of 'difficult')
This is the entire clue.
(Other definitions for hardest that I've seen before include "Trashed (anag) - most oppressive" , "Most difficult or firm" , "Red hats (anag.)" , "most difficult to unravel" .)