Grass or nettle (4)
I believe the answer is:
nark
I believe this is a double definition.
'grass' is the first definition.
(I've seen this before)
'nettle' is the second definition.
('nark' can be a synonym of 'nettle')
'or' is the link.
(Other definitions for nark that I've seen before include "Bug" , "Stool pigeon" , "Keep an eye" , "Annoy; police informer" , "Annoy; informer (sl.)" .)