Sauce is something unwelcome on duck (5)
I believe the answer is:
pesto
'sauce' is the definition.
(I've seen this before)
'something unwelcome on duck' is the wordplay.
'something unwelcome' becomes 'pest'.
'on' is a charade indicator (letters next to each other).
'duck' becomes 'o'.
'pest'+'o'='PESTO'
'is' is the link.
(Other definitions for pesto that I've seen before include "Pasta sauce mainly of basil and cheese" , "sauce involving Basil" , "Pasta sauce variety" , "Sauce made of basil and cheese" , "Basil-based sauce" .)