Six since returned passport endorsement (4)
I believe the answer is:
visa
'passport endorsement' is the definition.
(I've seen this in another clue)
'six since returned' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'since' becomes 'as' (both can mean 'because').
'returned' says the letters should be written in reverse.
'as' in reverse letter order is 'sa'.
'vi'+'sa'='VISA'
(Other definitions for visa that I've seen before include "travel authority" , "It allows the bearer to enter the country issuing it" , "Endorsement allowing bearer to enter country" , "Stamp affixed to passport as permission to enter country" , "Travel warrant" .)