Not able to tear a spell (7)
I believe the answer is:
cantrip
'a spell' is the definition.
(I've seen this before)
'not able to tear' is the wordplay.
'not able to' becomes 'can't' (cannot).
'tear' becomes 'rip' (synonyms).
'cant'+'rip'='CANTRIP'
(Other definitions for cantrip that I've seen before include "bit of mischief" , "Witch's trick" , "(Scottish spelling)" , "Magic spell" , "Piece of mischief" .)