Quiet during mass, perhaps, but there is an outburst (5)
I believe the answer is:
spasm
'there is an outburst' is the definition.
(I've seen this in another clue)
'quiet during mass perhaps' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'during' is an insertion indicator.
'perhaps' is an anagram indicator.
'mass' is an anagram of 'sasm'.
'p' put into 'sasm' is 'SPASM'.
'but' acts as a link.
(Other definitions for spasm that I've seen before include "Sudden muscular contraction" , "Fit" , "Burst" , "Tic, sudden contraction" , "Muscle twitch" .)