British tree with start of Dutch tree rot (10)
I believe the answer is:
balderdash
'rot' is the definition.
(thesaurus)
'british tree with start of dutch tree' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'tree' becomes 'alder' (alder is a kind of tree**).
'with' is a charade indicator (letters next to each other).
'start of' suggests taking the first letters.
'tree' becomes 'ash' (ash is a kind of tree).
The initial letter of 'dutch' is 'd'.
'b'+'alder'+'d'+'ash'='BALDERDASH'
(Other definitions for balderdash that I've seen before include "Twaddle" , "bull" , "Senseless talk/writing" , "Piffle" , "Nonsense" .)