Vagrant parking behind public transport (5)
I believe the answer is:
tramp
'vagrant' is the definition.
(I've seen this in another clue)
'parking behind public transport' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'behind' is a charade indicator (letters next to each other).
'public transport' becomes 'tram' (type of public transport).
'p' 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" .)