Served up duck with potato dressing (4)
I believe the answer is:
mayo
'dressing' is the definition.
(I've seen this before)
'served up duck with potato' is the wordplay.
'served up' says the letters should be written backwards.
'duck' becomes 'o'.
'with' is a charade indicator (letters next to each other).
'potato' becomes 'yam' (I have seen 'Sweet potato' mean 'yam' so perhaps 'potato' could also mean 'yam').
'o'+'yam'='oyam'
'oyam' reversed gives 'MAYO'.
(Other definitions for mayo that I've seen before include "Saucy thing" , "Irish county; dressing" , "Irish county (county town, Castlebar)" , "Irish county -- salad cream" , "Irish county - sauce (abbr)" .)