Tree on corner with rot (7)
I believe the answer is:
hogwash
'rot' is the definition.
('hogwash' can be a synonym of 'rot')
'tree on corner with' is the wordplay.
'tree' becomes 'ash' (ash tree is a kind of tree).
'on' says to put letters next to each other.
'corner' becomes 'hog' (I've seen this in other clues).
'with' becomes 'w' (abbreviation**).
'hog'+'w'='hogw'
'ash' put after 'hogw' is 'HOGWASH'.
(Other definitions for hogwash that I've seen before include "tripe" , "Pigswill - nonsense!" , "Pig food" , "Balderdash - pigswill" , "Bunk" .)