Land originally explored regularly in public transport (7)
I believe the answer is:
terrain
'land' is the definition.
(terrain is land or territory)
'originally explored regularly in public transport' is the wordplay.
'originally' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'public transport' becomes 'train' (train is a kind of public transport).
The first letters of 'explored regularly' is 'er'.
'er' put within 'train' is 'TERRAIN'.
(Other definitions for terrain that I've seen before include "(Type of) ground" , "Piece of ground" , "Stretch of land, emphasising natural features" , "A stretch of land for trainer" , "Tract of land of a specific character" .)