Let in a trailer with German attachment (5)

I believe the answer is:

admit

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'let in' is the definition.
(synonyms)

'a trailer with german attachment' is the wordplay.
'a trailer' becomes 'ad' (short for advert).
'with german attachment' becomes 'mit' (I can't explain this - if you can you should believe this answer much more).
'ad'+'mit'='ADMIT'

Can you help me to learn more?

(Other definitions for admit that I've seen before include "Allow access" , "Reveal" , "receive" , "Confess (to something)" , "Confess and permit entrance" .)

I've seen this clue in The Globe and Mail.

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