Liberal introducing novel proposition (5)
I believe the answer is:
lemma
'proposition' is the definition.
(lemma is a kind of proposition)
'liberal introducing novel' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'introducing' says to put letters next to each other.
'novel' becomes 'emma' (I've seen this before**).
'l'+'emma'='LEMMA'
(Other definitions for lemma that I've seen before include "heading" , "Intermediate proof" , "Subject's division" , "Proposition" , "Subsidiary theorem" .)