Tree planted in April in Denmark (6)
I believe the answer is:
linden
'tree' is the definition.
(linden is a kind of tree)
'planted in april in denmark' is the wordplay.
'planted in' indicates a hidden word.
'LINDEN' is hidden within 'april in denmark'.
(Other definitions for linden that I've seen before include "Tree with Lea - also called 8" , "type of tree" , "Unter den ------, iconic boulevard in Berlin" , "One twigged" , "Lime tree" .)