Tree finally planted during party? Nonsense (10)
I believe the answer is:
Here is my best explanation:
'nonsense' is the definition.
(balderdash is a kind of nonsense)
'tree finally planted during party?' is the subsidiary indication.
I cannot really understand how this works, but
'tree' could be 'alder' (alder is a kind of tree) and 'alder' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for balderdash that I've seen before include "Bunk", "Twaddle", "tripe", "Claptrap", "Poppycock", "cobblers > GAME", "Piffle", "Cobblers", "rot" and "Senseless talk".)