Pitfall about motorway for a long walk (5)
I believe the answer is:
tramp
'a long walk' is the definition.
(I've seen this before)
'pitfall about motorway' is the wordplay.
'pitfall' becomes 'trap' (I've seen this in another clue).
'about' is an insertion indicator.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'trap' going around 'm' is 'TRAMP'.
'for' acts as a link.
(Other definitions for tramp that I've seen before include "Merchant ship" , "cargo-boat" , "Derelict - ship" , "Beggar or vagrant" , "Walk with heavy steps like hobo" .)