Tree finally planted during party? Nonsense (10)
I believe the answer is:
balderdash
'nonsense' is the definition.
(balderdash is a kind of nonsense)
'tree finally planted during party?' is the wordplay.
I cannot quite see how this works, but
'tree' could be 'alder' (alder is a kind of tree) and 'alder' is located in the answer.
This explanation may well be incorrect...
(Other definitions for balderdash that I've seen before include "rot" , "Claptrap" , "Senseless talk" , "Piffle" , "tripe" .)