Twitch quietly during mass, possibly (5)
I believe the answer is:
spasm
'twitch' is the definition.
(I've seen this before)
'quietly during mass possibly' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'during' is an insertion indicator.
'possibly' is an anagram indicator.
'mass' is an anagram of 'sasm'.
'p' going within 'sasm' is 'SPASM'.
(Other definitions for spasm that I've seen before include "cramp" , "sudden movement" , "Involuntary muscular movement" , "Painful muscle contraction" , "Sudden convulsion" .)