Bird once spotted in holm oak (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 am not sure about the 'once' bit.
'spotted in holm oak' is the wordplay.
'spotted in' indicates the answer is hidden within the clue.
'MOA' is hidden within 'holm oak'.
Can you help me to learn more?
(Other definitions for moa that I've seen before include "was hunted by Maori" , "Bird lost" , "Bird's gone for ever" , "Gigantic extinct bird" , "bird that became extinct" .)