Terrible hag snorted, being wilful (10)
I believe the answer is:
headstrong
'being wilful' is the definition.
('headstrong' can be a synonym of 'wilful')
'terrible hag snorted' is the wordplay.
'terrible' indicates anagramming the letters.
'hag'+'snorted'='hagsnorted'
'hagsnorted' anagrammed gives 'HEADSTRONG'.
(Other definitions for headstrong that I've seen before include "Wilful, determined" , "Showing wilfulness" , "pig-headed" , "Obstinate" , "Stubborn" .)