Duck always found around one delta (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a kind of duck)
'always found around one delta' is the wordplay.
'always' becomes 'eer' (poetic way of saying ever).
'found around' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'i'+'d'='id'
'eer' going around 'id' is 'EIDER'.
(Other definitions for eider that I've seen before include "Duck supplying down" , "Duck noted for down" , "ie Red (anag.)" , "Duck valued for its fine soft down" , "Downy sea duck" .)