Walk quietly, following vehicle (5)
I believe the answer is:
tramp
'walk' is the definition.
(tramp is a kind of walk)
'quietly following vehicle' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'following' means one lot of letters go next to another.
'vehicle' becomes 'tram' (tram is a kind of vehicle).
'p' after 'tram' is 'TRAMP'.
(Other definitions for tramp that I've seen before include "cargo boat" , "Down-and-out" , "Traveller" , "Itinerant" , "Walk heavily - steamer" .)