Deer curiously circling one duck (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a kind of duck)
'deer curiously circling one' is the wordplay.
'curiously' indicates anagramming the letters.
'circling' indicates putting letters inside (circling can mean going around or containing).
'one' becomes 'i' (Roman numeral).
'deer' is an anagram of 'eder'.
'eder' going around 'i' is 'EIDER'.
(Other definitions for eider that I've seen before include "Breed of duck" , "Sea-duck" , "Duck valued for its fine soft down" , "Duck known for down" , "Type of duck" .)