Monsieur finishes repairs (5)
I believe the answer is:
'repairs' is the definition.
(I know this)
'monsieur finishes' is the wordplay.
'monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'finishes' becomes 'ends' ('end' can be a synonym of 'finish').
(Other definitions for mends that I've seen before include "Repairs, darns" , "Fixes" .)