Act under compulsion to cancel church Mass (7)
I believe the answer is:
perform
'act under compulsion to cancel church mass' is the definition.
I can't tell whether this defines the answer.
'act under compulsion to cancel church mass' is the wordplay.
'act under' becomes 'per' (I can't explain this - if you can you should believe this answer much more).
'compulsion' becomes 'force' (I've seen this before).
'to cancel' is a deletion indicator.
'church' becomes 'CE' (abbreviation for Church of England).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'force' with 'ce' taken out is 'for'.
'per'+'for'+'m'='PERFORM'
(Other definitions for perform that I've seen before include "Act - accomplish" , "Act in play or film" , "seen to make a profit?" , "Carry out; act" , "Carry out, accomplish" .)