Crossword Genius

A muscle unaffected by retraction (7)

Ross

I believe the answer is:

rotator

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Globe and Mail.
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