A muscle unaffected by retraction (7)
I believe the answer is:
rotator
'a muscle' is the definition.
(I've seen this before)
'unaffected by retraction' is the wordplay.
I cannot really understand how this works, but
'by' could be 'at' and 'at' is present in the answer.
The remaining letters 'rotor' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rotator that I've seen before include "Muscle that revolves a part of the body on its axis" , "Something that turns" , "It swivels" , "Muscle that turns" , "Revolver" .)