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' says to put letters next to each other.
'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" .)