Not able to rest in peace, with Scottish witch's spell (7)
I believe the answer is:
cantrip
'scottish witch's spell' is the definition.
(Scottish term for a trick)
'not able to rest in peace' is the wordplay.
'not able to' becomes 'can't' (cannot).
'rest in peace' becomes 'RIP' (inscription written on gravestones).
'cant'+'rip'='CANTRIP'
'with' acts as a link.
(Other definitions for cantrip that I've seen before include "Magical spell" , "Piece of mischief" , "bit of mischief" , "Magic spell" , "Witch's trick" .)