Thieves start to swindle vagrant (5)
I believe the answer is:
tramp
'vagrant' is the definition.
(I know that tramp is a type of vagrant)
'thieves start to swindle' is the wordplay.
'start to' suggests taking the first letters.
'swindle' becomes 'ramp' (I've seen this before).
The first letter of 'thieves' is 't'.
't'+'ramp'='TRAMP'
(Other definitions for tramp that I've seen before include "I usually walk" , "Bum!" , "Go heavily" , "Attack with knife" , "I might hitch a lift" .)