Against upsetting performance in part of church service (7)
I believe the answer is:
nocturn
'part of church service' is the definition.
'against upsetting performance' is the wordplay.
'against' becomes 'con' (I've seen this before).
'upsetting' shows that the letters should be reversed in order.
'performance' becomes 'turn' (turn is a kind of performance).
'con' backwards is 'noc'.
'noc'+'turn'='NOCTURN'
'in' acts as a link.
(Other definitions for nocturn that I've seen before include "Section of RC Matins" , "part of the office" , "part of service" .)