A road enthusiast reversing, that's plain (6)
I believe the answer is:
tundra
'plain' is the definition.
(tundra is a kind of plain)
'a road enthusiast reversing' is the wordplay.
'road' becomes 'rd' (abbreviation).
'enthusiast' becomes 'nut' (I've seen this before).
'reversing' says the letters should be written in reverse.
'a'+'rd'+'nut'='ardnut'
'ardnut' reversed gives 'TUNDRA'.
'that's' is the link.
(Other definitions for tundra that I've seen before include "where you might see caribou" , "Area of permafrost" , "Cold plain" , "Sub-polar treeless zone" , "northern waste" .)