Perform in terror, twanging muscle (9)
I believe the answer is:
retractor
'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 "that springs back in reverse?" , "It must pull back" , "Muscle" , "One withdraws" , "Instrument" .)