There's many a tree to reduce to pulp (4)
I believe the answer is:
mash
'reduce to pulp' is the definition.
(I've seen this before)
'there's many a tree' is the wordplay.
'there's' says to put letters next to each other.
'many' becomes 'm' (**).
'a tree' becomes 'ash' (ash is a kind of tree).
'm'+'ash' is 'MASH'.
'to' acts as a link.
(Other definitions for mash that I've seen before include "Black comedy film set in Korean War" , "Crush; soft mixture" , "Crush into a soft, pulpy state" , "Crush, potatoes say" , "Soft, pulpy mass" .)