The most common and most likely answer to this crossword clue is the 7 letter word TANBARK
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Buddy explains!
The straight part
We know that "Mulch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mulch = tanbark.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 TANBARK mean?
TANBARK noun- The bark of the oak (or other trees) used as a source of tannin
- The spent bark used as a ground covering