Naughty child inside Blenheim Palace (3)
I believe the answer is:
imp
'naughty child' is the definition.
(I've seen this before)
'inside blenheim palace' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'IMP' is hidden within 'blenheim palace'.
(Other definitions for imp that I've seen before include "Mischievous child" , "Mischievous fairy No.1" , "Wicked spirit" , "Pixie" , "old-fashioned graft" .)