Main road is a bore (4)
I believe the answer is:
drag
I believe this clue is a double definition.
'main road' is the first definition.
(informal term for a road)
'a bore' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for drag that I've seen before include "Pull along with effort" , "Pull along forcefully" , "Draw - transvestite attire" , "Short smoke" , "Trail along the ground" .)