Start to like novel proposition (5)
I believe the answer is:
lemma
'proposition' is the definition.
(lemma is a kind of proposition)
'start to like novel' is the wordplay.
'start to' says to take the initial letters.
'novel' becomes 'emma' (I've seen this before).
The first letter of 'like' is 'l'.
'l'+'emma'='LEMMA'
(Other definitions for lemma that I've seen before include "Proposition" , "Subject's division" , "Subsidiary theorem" , "Intermediate proof" , "heading" .)