Setter's power as mischief-maker (3)
I believe the answer is:
imp
'mischief-maker' is the definition.
(I've seen this before)
'setter's power' is the wordplay.
'setter's' becomes 'I'm' (referring to the person who set the crossword).
'power' becomes 'p'.
'im'+'p'='IMP'
'as' acts as a link.
(Other definitions for imp that I've seen before include "Small devil or sprite" , "Emperor" , "old-fashioned graft" , "puck" , "pickle" .)