Occasionally spurned girl making proposition (7)
I believe the answer is:
premiss
'proposition' is the definition.
(I know that premiss is a type of posit)
'occasionally spurned girl' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'girl' becomes 'miss' (term of address for an unmarried woman).
The alternate letters of 'spurned' are 'pre'.
'pre'+'miss'='PREMISS'
'making' acts as a link.
(Other definitions for premiss that I've seen before include "A condition stipulated beforehand" , "It's assumed" , "Proposition" , "piece of logic" , "Assumption" .)