Mischievous out of self-interest (5)
I believe the answer is:
elfin
'mischievous' is the definition.
(I've seen this before)
'out of self-interest' is the wordplay.
'out of' indicates the answer is hidden within the clue.
'ELFIN' can be found hidden inside 'self interest'.
(Other definitions for elfin that I've seen before include "Small and fragile" , "Small, delicately-framed" , "Little, charming and naughty" , "Describing Santa's helpers" , "little character" .)