The stormy petrel could be an armful (6)
I believe the answer is:
fulmar
'the stormy petrel' is the definition.
(thesaurus)
'could be an armful' is the wordplay.
'could be' is an anagram indicator.
'armful' anagrammed gives 'FULMAR'.
(Other definitions for fulmar that I've seen before include "Heavy short-tailed polar bird" , "Bird of petrel family" , "Gull-like sea bird" , "Heavy short-tailed oceanic bird" , "Get an strange armful of this seabird" .)