Vagrant parking behind public transport (5)
I believe the answer is:
tramp
'vagrant' is the definition.
(I've seen this before)
'parking behind public transport' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'behind' says to put letters next to each other.
'public transport' becomes 'tram' (type of public transport).
'p' put after 'tram' is 'TRAMP'.
(Other definitions for tramp that I've seen before include "Heavily plod around" , "Walk heavily or wearily" , "Heavy footsteps" , "A boat; a vagrant" , "Vagabond, hobo" .)