Little troublemaker (3)
'little troublemaker' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for imp that I've seen before include "A small mischievous devil" , "Emperor" , "Sprite; add feathers to" , "Small mischievous sprite" , "Naughty demon" .)
(Other definitions for twerp that I've seen before include "Nincompoop" , "Nitwit" , "Pillock" , "Idiot" , "Twit" .)
(Other definitions for hellion that I've seen before include "Person fond of devilry, trouble-maker" , "Rowdy or mischievous person" .)