Walk onto transport quietly (5)
I believe the answer is:
tramp
'walk' is the definition.
(tramp can mean to walk around)
'onto transport quietly' is the wordplay.
'onto' means one lot of letters go next to another.
'transport' becomes 'tram' (method of transport).
'quietly' becomes 'p' (p for piano and quiet).
'tram' put next to 'p' is 'TRAMP'.
(Other definitions for tramp that I've seen before include "Lady or gentleman of the road or roving ship" , "March heavily like hobo" , "I might hitch a lift" , "Mendicant" , "Down-and-out" .)