Proposition left to Austen heroine? (5)
I believe the answer is:
lemma
'proposition' is the definition.
(lemma is a kind of proposition)
'left to austen heroine?' is the wordplay.
'left' becomes 'L' (common abbreviation).
'to austen heroine?' becomes 'emma' (I've seen this before).
'l'+'emma'='LEMMA'
(Other definitions for lemma that I've seen before include "Proposition" , "heading" , "Subsidiary theorem" , "Subject's division" , "Intermediate proof" .)