Heavy short-tailed oceanic bird (6)
I believe the answer is:
fulmar
'heavy short-tailed oceanic bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fulmar that I've seen before include "Gull-like sea bird" , "Heavy short-tailed oceanic bird" , "Get an strange armful of this seabird" , "Heavy short-tailed polar bird" , "Bird of petrel family" .)