Lack of interest in proven nuisance (5)
I believe the answer is:
ennui
'lack of interest' is the definition.
(I've seen this in another clue)
'in proven nuisance' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ENNUI' is hidden in the letters of 'proven nuisance'.
(Other definitions for ennui that I've seen before include "Feeling of weariness or boredom" , "French boredom and lassitude" , "Feeling when tired of life?" , "to make one list less when connected" , "What's tedious and instils such listlessness" .)