Swinish ogre hiding in tree (7)
I believe the answer is:
porcine
'swinish' is the definition.
(I know that swinish can be written as porcine)
'ogre hiding in tree' is the wordplay.
'ogre' becomes 'orc' (both are horrible fantasy creatures).
'hiding in' means one lot of letters goes inside another.
'tree' becomes 'pine' (pine is a kind of tree).
'orc' going inside 'pine' is 'PORCINE'.
(Other definitions for porcine that I've seen before include "Of or like a pig" , "Of pigs" , "Filthy" , "Resembling a pig" , "Swinish" .)