Tell senior churchman is losing power (6)
I believe the answer is:
Here is my best explanation:
'tell' is the definition.
(relating is a kind of telling)
'senior churchman is losing power' is the subsidiary indication.
'senior churchman' becomes 'prelate' (I've seen this before).
'is losing' is a deletion indicator.
'power' becomes 'p'.
'prelate' with 'p' taken out is 'relate'.
(Other definitions for relate that I've seen before include "Have to do with", "Recount", "Make a logical or casual connection", "Narrate or associate", "Make a logical or causal connection", "Do well; wave about", "Give a report or show a connection", "Tell - the marriage guidance people", "Narrate, tell in detail" and "Tell in Tralee".)