Left with novel proposition (5)
I believe the answer is:
lemma
'proposition' is the definition.
(mathematical proposition)
'left with novel' is the wordplay.
'left' becomes 'l' (common abbreviation).
'with' says to put letters next to each other.
'novel' becomes 'Emma' (novel by Jane Austen).
'l'+'emma'='LEMMA'
(Other definitions for lemma that I've seen before include "Subsidiary theorem" , "Proposition" , "heading" , "Subject's division" , "Intermediate proof" .)