The end of the world means papacy lose ground (10)
I believe the answer is:
apocalypse
'the end of the world' is the definition.
(I've seen this before)
'papacy lose ground' is the wordplay.
'ground' indicates anagramming the letters (grind the letters ).
'papacy'+'lose'='papacylose'
'papacylose' with letters rearranged gives 'APOCALYPSE'.
'means' is the link.
(Other definitions for apocalypse that I've seen before include "End of the world" , "Revelation" , "Catastrophic event" , "World's end" , "This last book" .)