Sea duck behind heard at regular intervals (5)
I believe the answer is:
eider
'sea duck' is the definition.
(eider is a kind of sea duck)
'behind heard at regular intervals' is the wordplay.
'at regular intervals' means one should take alternating letters.
'behind'+'heard'='behindheard'
The alternate letters of 'behindheard' are 'EIDER'.
(Other definitions for eider that I've seen before include "Species of duck" , "Type of duck" , "Down's source" , "Downy sea duck" , "Sea duck with soft down" .)