Public transport in shopping centre that's revolutionary (4)
I believe the answer is:
tram
'public transport' is the definition.
(type of public transport)
'shopping centre that's revolutionary' is the wordplay.
'shopping centre' becomes 'mart' (large shopping complex).
'that's revolutionary' is a reversal indicator.
'mart' back-to-front is 'TRAM'.
'in' acts as a link.
(Other definitions for tram that I've seen before include "Former public vehicle" , "Road vehicle running on tracks" , "Rail vehicle" , "Passenger vehicle on rails" , "Electric vehicle" .)