Passport stamp (4)
I believe the answer is:
visa
'passport stamp' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for visa that I've seen before include "Stamp affixed to passport as permission to enter country" , "travel authority" , "Passport stamp or well-known credit card" , "Endorsement made in a passport" , "It allows the bearer to enter the country issuing it" .)