Transport for vagrant mostly (4)
I believe the answer is:
Here is my best explanation:
'transport' is the definition.
(tram is a kind of transport)
'vagrant mostly' is the wordplay.
'vagrant' becomes 'tramp' (I've seen this before).
'mostly' means to remove the last letter.
'tramp' with its last letter removed is 'TRAM'.
'for' is the link.
(Other definitions for tram that I've seen before include "Vehicle running on rails in road", "An early form of Luas in Dublin transport", "Bus on rails", "Electrically driven transport", "Car on rails", "The Luas, say", "City transport", "It runs on rails", "Public transport vehicle" and "Street vehicle running on rails".)