Mischievous child occupying him perpetually (3)
I believe the answer is:
imp
'mischievous child' is the definition.
(imp can mean a misbehaving child)
'occupying him perpetually' is the wordplay.
'occupying' indicates the answer is hidden within the clue (letters for the answer occupy part of the clue).
'IMP' is hidden within 'him perpetually'.
(Other definitions for imp that I've seen before include "puck" , "A small mischievous devil" , "Small mischievous spirit" , "Naughty child" , "Small mischievous sprite" .)