Kid in Wellington is, first of all, a New Zealander (4)
I believe the answer is:
kiwi
'a new zealander' is the definition.
(informal term for someone from New Zealand)
'kid in wellington is first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'kid in wellington is' is 'KIWI'.
(Other definitions for kiwi that I've seen before include "Someone from New Zealand?" , "New Zealander's bird" , "Someone from Auckland?" , "Enzedder; flightless bird" , "New Zealand bird that does not fly" .)