Let in a Duke with German (5)
I believe the answer is:
admit
'let in' is the definition.
(synonyms)
'a duke with german' is the wordplay.
'duke' becomes 'd' (abbreviation).
'with german' becomes 'mit' ('with' in German).
'a'+'d'+'mit'='ADMIT'
(Other definitions for admit that I've seen before include "Usher in" , "receive" , "Recognise" , "Confess; allow in" , "Reveal" .)