Perform in terror, twanging muscle (9)
I believe the answer is:
Here is my best explanation:
'muscle' is the definition.
(I've seen this before)
'perform in terror twanging' is the wordplay.
'perform' becomes 'act' (acting is a kind of performing).
'in' means one lot of letters goes inside another.
'twanging' indicates an anagram.
'terror' anagrammed gives 'retror'.
'act' inserted inside '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".)