Nut leapt about when sullen (8)
I believe the answer is:
petulant
'sullen' is the definition.
(thesaurus)
'nut leapt about' is the wordplay.
'about' indicates an anagram.
'nut'+'leapt'='nutleapt'
'nutleapt' with letters rearranged gives 'PETULANT'.
'when' is the link.
(Other definitions for petulant that I've seen before include "Fractious, pouty" , "Easily irritated" , "Irritable, sullen" , "Querulous and demanding" , "Easily provoked" .)