Mystery writer Marsh (5)
I believe the answer is:
ngaio
'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 "foreign tree" , "- Marsh, NZ author" , "- Marsh, NZ detective novelist" , "- - Marsh, crime writer" , "wood" .)