Bird once in trench, having lost tail (3)
I believe the answer is:
moa
'bird once' is the definition.
'moa' can be an answer for 'bird' (extinct flightless bird of New Zealand). I'm not sure about the 'once' bit.
'trench having lost tail' is the wordplay.
'trench' becomes 'moat' (moat is a kind of trench).
'having lost tail' means to remove the last letter.
'moat' with its final letter taken away is 'MOA'.
'in' is the link.
Can you help me to learn more?
(Other definitions for moa that I've seen before include "Bird's gone for ever" , "Bird lost" , "Extinct large flightless NZ bird" , "bird that became extinct" , "was hunted by Maori" .)