The most common and most likely answer to this crossword clue is the 4 letter word PULP
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The straight part
We know that "Mash" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mash = pulp.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PULP mean?
PULP noun- A soft, moist, shapeless mass or matter.
- A magazine or book containing lurid subject matter and characteristically printed on rough, unfinished paper.
- To make or be made into pulp.
- To beat to a pulp.
- To deprive of pulp; to separate the pulp from.
- Of or pertaining to pulp magazines; in the style of a pulp magazine or the material printed within such a publication.