Tell senior churchman is losing power (6)
I believe the answer is:
relate
'tell' is the definition.
(relating is a kind of telling)
'senior churchman is losing power' is the wordplay.
'senior churchman' becomes 'prelate' (I've seen this before).
'is losing' indicates named letters should be taken away.
'power' becomes 'p'.
'prelate' with 'p' taken away is 'RELATE'.
(Other definitions for relate that I've seen before include "Give a report or show a connection" , "Make a logical or casual connection" , "Tell in Tralee" , "Connect in Tralee" , "Give account of (story)" .)