In religious residence, sex is a pressing matter (8)
I believe the answer is:
priority
'a pressing matter' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'in religious residence sex' is the wordplay.
'in' means one lot of letters goes inside another.
'religious residence' becomes 'priory' (priory is a kind of religious residence).
'sex' becomes 'it' (a common euphemism).
'priory' placed around 'it' is 'PRIORITY'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for priority that I've seen before include "Status establishing relative importance or urgency" , "Matter of greatest importance" , "Precedence in rank, the right to be first" , "The right to go first" , "Something regarded as more important than others" .)