A moneylender might, but it won't appear in his books (4,8)
I believe the answer is:
love interest
'a moneylender might but it won't appear' is the definition.
I can't tell whether this defines the answer.
'his books' is the wordplay.
I cannot really see how this works, but
'books' could be 'nt' (New Testament) and 'nt' is found within the answer.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Another definition for love interest that I've seen is " Romantic subplot in play or film".)