Reduce to pulp (4)
I believe the answer is:
'reduce to pulp' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mash that I've seen before include "Crush into a soft, pulpy state" , "Pummel" , "Accompaniment of bangers" , "Pulped potatoes" , "Sham sort of pulp" .)