Duck as darn Frenchman comes in (4)
I believe the answer is:
smew
'duck' is the definition.
(smew is a kind of duck)
'darn frenchman comes in' is the wordplay.
'darn' becomes 'sew' (I've seen this in another clue).
'frenchman' becomes 'm' (abbreviation for Monsieur).
'comes in' means one lot of letters goes inside another.
'sew' placed around 'm' is 'SMEW'.
'as' is the link.
(Other definitions for smew that I've seen before include "Bird" , "Species of merganser" , "Sea duck" , "Diver" , "Small merganser" .)