Unpleasant bird on the air (4)
I believe the answer is:
foul
'unpleasant' is the definition.
(synonyms)
'bird on the air' is the wordplay.
'bird' becomes 'fowl' (synonyms).
'on the air' shows a homophone (sound like).
'fowl' is a homophone of 'FOUL'.
(Other definitions for foul that I've seen before include "A breach of sporting rules" , "Break the rules on the pitch" , "Loathsome - tackle breaking game rules" , "Obscene" , "Putrid" .)