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