Former pope's mountain bull
I believe the answer is:
benedict
'former pope's' is the definition.
'benedict' can be an answer for 'pope's' (I have seen 'Present Pope ' mean 'benedict' so perhaps 'pope' could also mean 'benedict'). I am not certain of the 'former' bit.
'mountain bull' is the wordplay.
'mountain' becomes 'ben' (ben is a kind of mountain).
'bull' becomes 'edict' (as in a papal bull, an edict from the pope).
'ben'+'edict'='BENEDICT'
Can you help me to learn more?
(Other definitions for benedict that I've seen before include "Arnold, for example" , "Saint followed by an order of monks" , "-- Cumberbatch, actor" , "See chief once" , "Name of current Pope" .)