There's many a tree to reduce to pulp (4)
I believe the answer is:
Here is my best explanation:
'reduce to pulp' is the definition.
(I've seen this before)
'there's many a tree' is the subsidiary indication.
'there's' says to put letters next to each other.
'many' becomes 'm'.
'a tree' becomes 'ash' (ash is a kind of tree).
'm' put next to 'ash' is 'mash'.
'to' is the link.
(Other definitions for mash that I've seen before include "It goes well with sausages", "Pulped potatoes", "Bangers' frequent partner?", "Animal feed", "Sausages' traditional partner", "Crush into a soft, pulpy state", "Reduce to pulp", "Pulverise", "Sham sort of pulp" and "(Crush into) pulp".)