Things given up when renouncing Catholic fellow admitted (11)

'things' is the definition.
The answer and definition can be both acts as well as being plural nouns.
Maybe there's an association between them I don't understand?

'up when renouncing catholic fellow admitted' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is located in the answer.
'catholic' could be 'c' (abbreviation as in RC for Roman Catholic) and 'c' is located in the answer.
'fellow' could be 'f' (abbreviation - of a society etc.) and 'f' is found within the answer.
This explanation may well be incorrect...

'given' is the link.

Can you help me to learn more?