Trees in Wilhelmstrasse (4)
I believe the answer is:
elms
'trees' is the definition.
(elm is a kind of tree)
'in wilhelmstrasse' is the wordplay.
'in' indicates a hidden word.
'ELMS' is hidden within 'wilhelmstrasse'.
(Other definitions for elms that I've seen before include "Deciduous trees with rough serrated leaves" , "Trees sound like Cockney steering devices" , "Trees (of late, prone to disease)" , "Mel's strange trees" , "may have many leaves" .)