Proposition from Frenchman, then married in the country (5)
I believe the answer is:
lemma
'proposition' is the definition.
(lemma is a kind of proposition)
'frenchman then married in the country' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'then' is a charade indicator (letters next to each other).
'married' becomes 'm'.
'in' means one lot of letters goes inside another.
'the country' becomes 'lea' (I've seen this before).
'm'+'m'='mm'
'mm' going inside 'lea' is 'LEMMA'.
'from' acts as a link.
(Other definitions for lemma that I've seen before include "Subject's division" , "Proposition" , "Subsidiary theorem" , "heading" , "Intermediate proof" .)