Keen on independent books on love (4)
I believe the answer is:
into
'keen on' is the definition.
(I've seen this before)
'independent books on love' is the wordplay.
'independent' becomes 'i'.
'books' becomes 'nt' (New Testament).
'on' says to put letters next to each other.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'i'+'nt'+'o'='INTO'
(Other definitions for into that I've seen before include "Teachers' trade union" , "Liking" , "Keen on, colloquially" , "Four this twelve is three" , "Knowledgeable about" .)