Unpleasant bird, sea eagle flying about (12)
I believe the answer is:
disagreeable
'unpleasant' is the definition.
('disagreeable' can be similar in meaning to 'unpleasant')
'bird sea eagle flying about' is the wordplay.
'flying about' is an anagram indicator.
'bird'+'sea'+'eagle'='birdseaeagle'
'birdseaeagle' with letters rearranged gives 'DISAGREEABLE'.
(Other definitions for disagreeable that I've seen before include "Bad-tempered" , "Unpleasant" , "Obnoxious" , "Offensive - peevish" .)