Clergyman beheaded immediately (4)
I believe the answer is:
anon
'immediately' is the definition.
(thesaurus)
'clergyman beheaded' is the wordplay.
'clergyman' becomes 'canon' (canon is a kind of clergyman).
'beheaded' means to remove the first letter.
'canon' with its initial letter taken off is 'ANON'.
(Other definitions for anon that I've seen before include "Soon - nobody knows me" , "Soon, in a little while" , "coming after a call" , "In a little while (informal)" , "Shortly; unknown" .)