After eating duck, diner's fancy decreased? (6)
I believe the answer is:
ironed
'decreased?' is the definition.
(I've seen this before)
'after eating duck diner's fancy' is the wordplay.
'after eating' means one lot of letters goes inside another.
'duck' becomes 'o'.
'fancy' is an anagram indicator.
'diner' anagrammed gives 'irned'.
'o' going into 'irned' is 'IRONED'.
(Other definitions for ironed that I've seen before include "Smoothed clothes using heavy object" , "Made smooth, shirt or collar" , "Had an evening out" , "Smoothed the cloth with heat" , "Smoothed with domestic device" .)