Vagrants heading for prom in streetcars (6)
I believe the answer is:
tramps
'vagrants' is the definition.
(I've seen this before)
'heading for prom in streetcars' is the wordplay.
'heading for' says to take the initial letters.
'in' is an insertion indicator.
'streetcars' becomes 'trams' ('tram' can be a synonym of 'streetcar').
The initial letter of 'prom' is 'p'.
'p' placed within 'trams' is 'TRAMPS'.
(Other definitions for tramps that I've seen before include "Become hard or solid" , "Homeless people" , "Goes on a long walk" , "Journeys wearily" , "Walks heavily like hobos" .)