Concealed inside, the bird did damage (6)
I believe the answer is:
hidden
'concealed' is the definition.
(I know that concealed can be written as hidden)
'inside the bird did damage' is the wordplay.
'inside' indicates putting letters inside.
'the bird' becomes 'hen' (hen is a kind of bird).
'damage' indicates an anagram (I've seen 'damaged' mean this).
'did' with letters rearranged gives 'idd'.
'hen' enclosing 'idd' is 'HIDDEN'.
(Other definitions for hidden that I've seen before include "not obvious" , "Concealed, kept secret" , "Unseen" , "Obscure" , "Abstruse" .)