Check alien showing a means to gain entry (6)

I believe the answer is:

ticket

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'a means to gain entry' is the definition.
(eg a ticket for an event)

'check alien' is the wordplay.
'check' becomes 'tick' (I can't justify this - if you can you should give a lot more credence to this answer).
'alien' becomes 'et' (extra terrestrial).
'tick'+'et'='TICKET'

'showing' acts as a link.

Can you help me to learn more?

(Other definitions for ticket that I've seen before include "Entry/travel permit" , "Token for admission" , "Voucher giving entry" , "Pass - docket" , "Pass or permit" .)

I've seen this clue in The Guardian.

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