In writings, love is something that grows (4)
I believe the answer is:
moss
'in writings love is something that grows' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'in writings love' is the wordplay.
'in' means one lot of letters goes inside another.
'writings' becomes 'mss' (more than one MS ie manuscript).
'love' becomes 'o' (love means zero in tennis).
'mss' placed around 'o' is 'MOSS'.
Can you help me to learn more?
(Other definitions for moss that I've seen before include "Veteran racing driver" , "Plant(s) not wanted on lawns" , "Former racing driver" , "Spore-producing plant" , "Supermodel Kate, b.1974" .)