Arm muscle -- it turns in both directions (7)
I believe the answer is:
rotator
This clue could be a double definition.
'arm muscle' is the first definition.
'rotator' can be an answer for 'muscle' (I've seen this in another clue). I'm not certain of the 'arm' bit.
'it turns in both directions' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for rotator that I've seen before include "anatomical drawing may reveal such muscle" , "It swivels" , "Device that spins" , "One palindromic turner ..." , "One goes round" .)