Mark finds alien permit (6)
I believe the answer is:
ticket
'permit' is the definition.
(I've seen this before)
'mark finds alien' is the wordplay.
'mark' becomes 'tick' (I've seen this before**).
'finds' means one lot of letters go next to another.
'alien' becomes 'et' (extra terrestrial).
'tick'+'et'='TICKET'
(Other definitions for ticket that I've seen before include "Parking offence notice" , "Buy one for admittance" , "needed for train journey" , "Voucher giving entry" , "Tag showing fare or fee" .)