Word of love harboured by religious woman (4)
I believe the answer is:
noun
'word' is the definition.
(noun is a kind of word)
'love harboured by religious woman' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'harboured by' is an insertion indicator.
'religious woman' becomes 'nun' (I've seen this before).
'o' inserted inside 'nun' is 'NOUN'.
'of' acts as a link.
(Other definitions for noun that I've seen before include "Part of speech identifying a person, place or thing" , "Naming word (grammar)" , "Category of word" , "Content word for person, place, thing etc" , "Word identifying person, place or thing" .)