Young lady's on before proposition (7)
I believe the answer is:
premiss
'proposition' is the definition.
(premiss is a kind of proposition)
'young lady's on before' is the wordplay.
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady').
'on' says to put letters next to each other.
'before' becomes 'pre'.
'miss' after 'pre' is 'PREMISS'.
(Other definitions for premiss that I've seen before include "A condition stipulated beforehand" , "Assumption" , "Proposition" , "piece of logic" , "It's assumed" .)