Passenger bill includes rodent (6)
I believe the answer is:
gerbil
'rodent' is the definition.
(I know that gerbil is a type of rodent)
'passenger bill includes' is the wordplay.
'includes' says the answer is hidden in the clue.
'GERBIL' is hidden within 'passenger bill'.
(Other definitions for gerbil that I've seen before include "Mouse-like desert rodent with long hind legs" , "Mouse-like rodent (G not J)" , "Small burrowing rodent" , "Rodent; rib leg (anag.)" , "Burrowing mouse-like rodent" .)