New Zealand mystery writer Marsh (5)
I believe the answer is:
ngaio
'new zealand mystery writer marsh' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ngaio that I've seen before include "- Marsh, NZ author" , "foreign tree" , "New Zealand tree" , "NZ tree; in Goa (anag.)" , "wood" .)