Passenger maybe arresting a looter (6)
I believe the answer is:
raider
'looter' is the definition.
(I know that looter can be written as raider)
'passenger maybe arresting a' is the wordplay.
'passenger maybe' becomes 'rider' (eg someone riding a train).
'arresting' indicates putting letters inside.
'rider' placed around 'a' is 'RAIDER'.
(Other definitions for raider that I've seen before include "One making incursion" , "person who's involved in sacking?" , "Viking" , "looter" , "Invader" .)