O Christ! That could be a bird (7)
I believe the answer is:
ostrich
'a bird' is the definition.
(ostrich is a kind of bird)
'o christ that could be' is the wordplay.
'that could be' indicates anagramming the letters.
'christ' is an anagram of 'strich'.
'o'+'strich'='OSTRICH'
(Other definitions for ostrich that I've seen before include "Emu-like bird" , "He ignores the truth" , "with a big nest egg" , "source of showy feathers" , "non-flyer?" .)