Plant of historic interest on the moor (5)
I believe the answer is:
sedge
'plant of' is the definition.
'sedge' can be an answer for 'plant' (I've seen this before). I'm not certain of the 'of' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for sedge that I've seen before include "Wetland grass-like plant" , "Bog plant; type of warbler" , "Plant growing in swampy ground" , "Marsh grass" , "water-loving vegetation" .)