Marsh is often seen by this tree (5)
I believe the answer is:
ngaio
This clue is a double definition.
'marsh is often seen by this' is the first definition.
(as in crime writer Ngaio Marsh)
'tree' is the second definition.
(I've seen this before)
(Other definitions for ngaio that I've seen before include "New Zealand tree" , "- - Marsh, crime writer" , "NZ tree; in Goa (anag.)" , "- Marsh, NZ detective novelist" , "foreign tree" .)