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