Lack of interest in sullen nuisance? (5)
I believe the answer is:
ennui
'lack of interest' is the definition.
(I've seen this before)
'in sullen nuisance?' is the wordplay.
'in' indicates a hidden word.
'ENNUI' can be found hidden inside 'sullen nuisance'.
(Other definitions for ennui that I've seen before include "boring" , "Listless feeling" , "Weariness with life" , "Listless boredom" , "having nothing to do" .)