Indigo American duck (4)
I believe the answer is:
anil
'indigo' is the definition.
(anil is another word for indigo)
'american duck' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'duck' becomes 'nil' (both can mean zero).
'a'+'nil'='ANIL'
(Other definitions for anil that I've seen before include "A blue dye" , "West Indian shrub, source of indigo" , "Shrub providing indigo" , "Dye for nail" , "It produces dye" .)