Deer upset about one duck (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a type of duck)
'deer upset about one' is the wordplay.
'upset' is an anagram indicator.
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'deer' anagrammed gives 'eder'.
'eder' enclosing 'i' is 'EIDER'.
(Other definitions for eider that I've seen before include "Duck valued for its fine soft down" , "Down comes from this" , "Duck from which down is obtained" , "Sea duck with soft down" , "Variety of duck" .)