Working in transport to get extra income (5)
I believe the answer is:
bonus
'extra income' is the definition.
(I've seen this before)
'working in transport' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'in' means one lot of letters goes inside another.
'transport' becomes 'bus' (bus is a kind of transport).
'on' inserted into 'bus' is 'BONUS'.
'to get' acts as a link.
(Other definitions for bonus that I've seen before include "Good-performance payment" , "Premium" , "Additional incentive payment" , "Additional payment or benefit above the norm" , "Welcome and unexpected extra" .)