They're in the Wilhelmstrasse, not the Unter den Linden (4)
I believe the answer is:
elms
'not the unter den linden' is the definition.
I can't tell whether this definition defines the answer.
'they're in the wilhelmstrasse' is the wordplay.
'ELMS' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for elms that I've seen before include "may have many leaves" , "rugged feature of country churchyard" , "Deciduous trees with rough serrated leaves" , "Mel's strange trees" , "Trees sound like Cockney steering devices" .)