… a lot of trees waste away, going to pot (7)

'pot' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'a lot of trees waste away going' is the wordplay.
'a lot of' means to remove the last letter (a lot of but not all of the word).
'trees' becomes 'pines' (pine is a kind of tree).
'waste away going' becomes 'tum' (I can't justify this - if you can you should give a lot more credence to this answer).
'pines' with its final letter taken off is 'pine'.
'pine'+'tum'='PINETUM'

'to' is the link.

Can you help me to learn more?