Terrible hag snorted, being wilful (10)
I believe the answer is:
headstrong
'wilful' is the definition.
(synonyms)
'terrible hag snorted' is the wordplay.
'terrible' is an anagram indicator.
'hag'+'snorted'='hagsnorted'
'hagsnorted' with letters rearranged gives 'HEADSTRONG'.
'being' acts as a link.
(Other definitions for headstrong that I've seen before include "Wilful, determined" , "Showing wilfulness" , "pig-headed" , "Obstinate" , "Stubborn" .)