Food for passenger (4)
I believe the answer is:
fare
This clue is a double definition.
'food' is the first definition.
(fare is a kind of food)
'passenger' is the second definition.
(fare is a kind of passenger)
'for' acts as a link.
(Other definitions for fare that I've seen before include "Taxi charge; food" , "(Person paying) price for travel" , "Food for the cost of a journey" , "Food - money for travelling - get on" , "Amount charged" .)