Little troublemaker (3)
'little troublemaker' is the definition.
(I've seen this in another clue)
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" .)