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 "Reduce to pulp", "Crush; soft mixture", "accompaniment", "Sausages' traditional partner", "Crush into a soft, pulpy state", "Crush, potatoes say", "dish of veg", "It goes well with sausages", "Pured potato" and "food for animals".)