Duck: one protected by wild deer (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a type of duck)
'one protected by wild deer' is the wordplay.
'one' becomes 'i' (Roman numeral).
'protected by' indicates putting letters inside.
'wild' indicates anagramming the letters.
'deer' with letters rearranged gives 'eder'.
'i' put into 'eder' is 'EIDER'.
(Other definitions for eider that I've seen before include "I've noted down" , "Variety of duck" , "Breed of duck" , "Duck known for down" , "Duck noted for down" .)