Bar that's shut up when clergyman enters (7)
I believe the answer is:
prevent
'bar' is the definition.
(I know that bar is a more specific form of the action block)
'shut up when clergyman enters' is the wordplay.
'shut up' becomes 'pen' (pen can mean to shut in or enclose).
'when' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'clergyman' becomes 'rev' (short for reverend).
'enters' means one lot of letters goes inside another.
'pen'+'t'='pent'
'pent' placed around 'rev' is 'PREVENT'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for prevent that I've seen before include "Keep from arising" , "Bar" , "Stop from happening" , "Stop, avert" , "Stop (an action)" .)