Tree died after onset of blight -- wood rot (10)
I believe the answer is:
balderdash
'rot' is the definition.
(I've seen this in another clue)
'tree died after onset of blight wood' is the wordplay.
'tree' becomes 'alder' (alder is a kind of tree).
'died' becomes 'd' (abbreviation next to the year of someone's death).
'after' is a charade indicator (letters next to each other).
'onset of' says to take the initial letters.
'wood' becomes 'ash' (wood from an ash tree).
The initial letter of 'blight' is 'b'.
'alder'+'d'='alderd'
'alderd' after 'b' is 'balderd'.
'balderd'+'ash'='BALDERDASH'
(Other definitions for balderdash that I've seen before include "stupid talk" , "Nonsense" , "rot" , "Senseless talk/writing" , "Bunk" .)