Trading centre set up public transport (4)
I believe the answer is:
tram
'public transport' is the definition.
(type of public transport)
'trading centre set up' is the wordplay.
'trading centre' becomes 'mart' (I've seen this before).
'set up' is a reversal indicator (in a down clue, letters go up).
'mart' in reverse letter order is 'TRAM'.
(Other definitions for tram that I've seen before include "Urban transport" , "Electric vehicle" , "Take it to the mart" , "It runs on rails" , "Rail vehicle" .)