They are mostly awfully coarse (6)
I believe the answer is:
earthy
'coarse' is the definition.
('earthy' can be a synonym of 'coarse')
'they are mostly awfully' is the wordplay.
'mostly' means to remove the last letter.
'awfully' indicates anagramming the letters.
'they'+'are'='theyare'
'theyare' with its last letter taken away is 'theyar'.
'theyar' anagrammed gives 'EARTHY'.
(Other definitions for earthy that I've seen before include "Head covering" , "Suggestive of soil" , "Coarse or gross" , "Dirty" , "Lusty, coarse" .)