Marsh of mysteries (5)
I believe the answer is:
ngaio
'marsh of mysteries' 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" , "New Zealand tree" , "- Marsh, NZ detective novelist" , "- - Marsh, crime writer" , "foreign tree" .)