Anger about bird boast concealed (7)
I believe the answer is:
umbrage
'anger' is the definition.
(I know that umbrage is a type of anger)
'bird boast concealed' is the wordplay.
I cannot quite see how this works, but
'boast' could be 'brag' (synonyms) and 'brag' is found within the answer.
This explanation may well be incorrect...
'about' acts as a link.
Can you help me to learn more?
(Other definitions for umbrage that I've seen before include "Take this to feel offence" , "It''s given or taken in offence" , "In high dudgeon" , "Offence that's usually taken" , "Offence, annoyance" .)