Miles to be covered in carriage, or long walk (5)
I believe the answer is:
tramp
'long walk' is the definition.
(I've seen this before)
'miles to be covered in carriage' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'to be covered in' is an insertion indicator.
'carriage' becomes 'trap' (trap is a kind of carriage).
'm' put inside 'trap' is 'TRAMP'.
'or' acts as a link.
(Other definitions for tramp that I've seen before include "Traveller" , "Walk with heavy steps like hobo" , "Footslog" , "Ship, possibly" , "Walk noisily; vagrant" .)