Perform in terror, twanging muscle (9)
I believe the answer is:
Here is my best explanation:
'muscle' is the definition.
'perform in terror twanging' is the subsidiary indication.
'perform' becomes 'act' (acting is a kind of performing).
'in' indicates putting letters inside.
'twanging' indicates an anagram.
'terror' with letters rearranged gives 'retror'.
'act' placed into 'retror' is 'retractor'.
(Other definitions for retractor that I've seen before include "It must pull back", "that springs back in reverse?", "Instrument", "theatre item", "One withdraws" and "Muscle".)