Bit of extra money for public transport (4)
I believe the answer is:
tram
'public transport' is the definition.
(type of public transport)
'bit of extra money for' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'TRAM' can be found hidden inside 'extra money for'.
(Other definitions for tram that I've seen before include "Vehicle running on rails in road" , "Bus on rails" , "Old city transport, ram on rails through streets" , "Electric vehicle" , "Take it to the mart" .)