A duck regularly considered to be a seabird (3)
I believe the answer is:
auk
'seabird' is the definition.
(auk is a kind of seabird)
'a duck regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'duck' are 'uk'.
'a'+'uk'='AUK'
'considered to be a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for auk that I've seen before include "Northern sea bird with short narrow wings" , "Black-and-white diving bird of northern seas" , "Great ---, extinct seabird" , "Northern seabird with small narrow wings" , "Diving sea bird" .)