Portion of flavourful marine bird (6)
I believe the answer is:
fulmar
'bird' is the definition.
(fulmar is a kind of bird)
'portion of flavourful marine' is the wordplay.
'portion of' says the answer is hidden in the clue.
'FULMAR' is hidden within 'flavourful marine'.
(Other definitions for fulmar that I've seen before include "Heavy short-tailed oceanic bird" , "Gull-like sea bird" , "Heavy short-tailed polar bird" , "Bird of petrel family" , "Get an strange armful of this seabird" .)