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' is a charade indicator (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" .)