Most difficult dart he's thrown (7)
I believe the answer is:
hardest
'most difficult' is the definition.
('hard' can be a synonym of 'difficult')
'dart he's thrown' is the wordplay.
'thrown' indicates anagramming the letters.
'dart'+'hes'='darthes'
'darthes' with letters rearranged gives 'HARDEST'.
(Other definitions for hardest that I've seen before include "Most difficult or firm" , "Trashed (anag) - most oppressive" , "Red hats (anag.)" , "most difficult to unravel" .)