Mark correct alien's permit (6)
I believe the answer is:
ticket
'permit' is the definition.
(I've seen this before)
'mark correct alien's' is the wordplay.
'mark correct' becomes 'tick' (to give an answer a tick is to mark it correct).
'alien' becomes 'et' (extra terrestrial).
'tick'+'et'='TICKET'
(Other definitions for ticket that I've seen before include "collector wants to view?" , "Slip showing payment of entrance fee" , "Entry/travel permit" , "needed for train journey" , "Slip showing payment of fee or fare" .)