Proposition regularly spurned by girl (7)
I believe the answer is:
premiss
'proposition' is the definition.
(premiss is a kind of proposition)
'regularly spurned by girl' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'by' says to put letters next to each other.
'girl' becomes 'miss' (I've seen this before**).
The alternating letters of 'spurned' are 'pre'.
'pre'+'miss'='PREMISS'
(Other definitions for premiss that I've seen before include "A condition stipulated beforehand" , "It's assumed" , "Assumption" , "Proposition" , "piece of logic" .)