Rot in heart of beech and another couple of trees (7)
I believe the answer is:
eyewash
'rot' is the definition.
(I've seen this before)
'heart of beech and another couple of trees' is the wordplay.
I cannot quite see how this works, but
'heart' could be 'eye' (synonyms) and 'eye' is found within the answer.
The remaining letters 'wash' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' is the link.
(Other definitions for eyewash that I've seen before include "Balderdash" , "Optic solution to relieve irritation - rubbish!" , "this is soothing sight" , "Visual cleansing agent - nonsense" , "Insincere talk or optical lotion" .)