Plant end of oar in marsh (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'end of oar in marsh' is the wordplay.
'end of' indicates one should take the final letters.
'in' is an insertion indicator.
'marsh' becomes 'fen' ('fen' can be a synonym of 'marsh').
The last letter of 'oar' is 'r'.
'r' going within 'fen' is 'FERN'.
(Other definitions for fern that I've seen before include "Non-flowering plant" , "Feathery frond plant" , "Plant with featherlike leaves" , "Type of plant" , "Often shady plant" .)