No good taking on excellent old New Zealand tree (5)
I believe the answer is:
ngaio
'new zealand tree' is the definition.
(I've seen this before)
'no good taking on excellent old' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'taking on' is a charade indicator (letters next to each other).
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'old' becomes 'o' (abbreviation).
'ng'+'ai'+'o'='NGAIO'
(Other definitions for ngaio that I've seen before include "NZ tree; in Goa (anag.)" , "- Marsh, NZ author" , "- Marsh, NZ detective novelist" , "New Zealand tree" , "foreign tree" .)