Proposition left a married French woman taken aback (5)
I believe the answer is:
lemma
'proposition' is the definition.
(mathematical proposition)
'left a married french woman taken aback' is the wordplay.
'left' becomes 'l' (common abbreviation).
'married' says to put letters next to each other.
'french woman' becomes 'Mme' (abbreviation for madame).
'taken aback' shows that the letters should be reversed in order.
'a'+'mme'='amme'
'amme' written backwards gives 'emma'.
'l'+'emma'='LEMMA'
(Other definitions for lemma that I've seen before include "Subsidiary theorem" , "heading" , "Proposition" , "Subject's division" , "Intermediate proof" .)