Backing rapid alternative to buses? (5)
I believe the answer is:
trams
'alternative to buses?' is the definition.
'backing rapid' is the wordplay.
'backing' says the letters should be written in reverse.
'rapid' becomes 'smart'.
'smart' in reverse letter order is 'TRAMS'.
(Other definitions for trams that I've seen before include "Not seen in London" , "People movers" , "old vehicles" , "silk yarns" , "public transport" .)