Prehistoric burial mounds (6)
I believe the answer is:
cairns
'prehistoric burial mounds' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cairns that I've seen before include "Heaps of stones - dogs" , "Prehistoric burial mounds made of stones" , "Stone mounds" , "Mounds of stones on mountain tops" , "Mound of stones" .)