Aquatic bird has tail bitten off? Goodness me (3)
I believe the answer is:
coo
'goodness me' is the definition.
(exclamation of surprise)
'aquatic bird has tail bitten off?' is the wordplay.
'aquatic bird' becomes 'coot' (coot is a kind of aquatic bird).
'has tail bitten off?' means to remove the last letter.
'coot' with its last letter removed is 'COO'.
(Other definitions for coo that I've seen before include "it's surprising!" , "Breathe" , "Sound coming from dovecote" , "Expression of surprise (by a dove?)" , "Bill and I go together" .)