Slapper clutching ends of grotesque butt (6)
I believe the answer is:
target
'butt' is the definition.
('target' can be a synonym of 'butt')
'slapper clutching ends of grotesque' is the wordplay.
'slapper' becomes 'tart' (British slang).
'clutching' indicates putting letters inside.
'ends of' means to remove the middle letters.
'grotesque' with its centre removed is 'ge'.
'tart' going around 'ge' is 'TARGET'.
(Other definitions for target that I've seen before include "Aimpoint" , "Thing aimed at" , "bull's eye" , "Something to aim for" , "Fixed goal" .)