English wood with tree rot (7)
I believe the answer is:
eyewash
'rot' is the definition.
(both can mean nonsense)
'english wood with tree' is the wordplay.
'english' becomes 'e' (abbreviation).
'wood' becomes 'yew' (yew is a kind of wood).
'with' means one lot of letters go next to another.
'tree' becomes 'ash' (ash is a kind of tree).
'e'+'yew'+'ash'='EYEWASH'
(Other definitions for eyewash that I've seen before include "Slangy nonsense or optical cleanser" , "Balderdash" , "Optic solution to relieve irritation - rubbish!" , "Optic lotion - nonsense!" , "Nonsense (informal)" .)