Heartless mistake about shock treatment for a muscle (7)
I believe the answer is:
erector
'muscle' is the definition.
(I've seen this before)
'heartless mistake about shock treatment' is the wordplay.
'heartless' suggests removing the centre.
'mistake' becomes 'error' (synonyms).
'about' is an insertion indicator.
'shock treatment' becomes 'ect' (ECT is an example).
'error' with its centre removed is 'eror'.
'eror' placed around 'ect' is 'ERECTOR'.
'for a' acts as a link.
(Other definitions for erector that I've seen before include "Constructor" , "Builder" , "The individual raising" , "puts things up" , "Muscle" .)