Let in a trailer with German attachment (5)
I believe the answer is:
admit
'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" .)