Deer stumbled about one duck (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a type of duck)
'deer stumbled about one' is the wordplay.
'stumbled' is an anagram indicator (I've seen 'stumbling' mean this).
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'deer' anagrammed gives 'eder'.
'eder' placed around 'i' is 'EIDER'.
(Other definitions for eider that I've seen before include "Down-producing duck" , "Duck noted for down" , "Species of duck" , "Duck known for down" , "I've noted down" .)