The most common and most likely answer to this crossword clue is the 7 letter word PABULUM
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The straight part
We know that "mush" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mush = pabulum.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 PABULUM mean?
PABULUM noun- Food or fodder, particularly that taken in by plants or animals.
- Material that feeds a fire.
- Food for thought.
- Bland intellectual fare; an undemanding diet of words.