Mischievous creature in the style of an antelope (6)
I believe the answer is:
impala
'an antelope' is the definition.
(impala is a kind of antelope)
'mischievous creature in the style of' is the wordplay.
'mischievous creature' becomes 'imp' (I've seen this before).
'in the style of' becomes 'ala' (I've seen this before).
'imp'+'ala'='IMPALA'
(Other definitions for impala that I've seen before include "Breed of antelope" , "leaping creature" , "Graceful antelope" , "Species of African antelope" , "One regularly browsing" .)