Extra payment on getting into public transport (5)
I believe the answer is:
bonus
'extra payment' is the definition.
(I've seen this before)
'on getting into public transport' is the wordplay.
'getting into' is an insertion indicator.
'public transport' becomes 'bus' (bus is a kind of public transport**).
'on' inserted into 'bus' is 'BONUS'.
(Other definitions for bonus that I've seen before include "Extra money paid for good performance" , "Extra payout" , "Premium" , "Additional incentive payment" , "Extra dividend" .)