The most common and most likely answer to this crossword clue is the 10 letter word WATERPROOF
I've seen this crossword clue in Independent
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More clues leading to the result WATERPROOF
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The straight part
We know that "Mac" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mac = waterproof.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 WATERPROOF mean?
WATERPROOF noun- A substance or preparation for rendering cloth, leather, etc., impervious to water.
- Cloth made waterproof, or any article made of such cloth, or of other waterproof material, as rubber; especially, an outer garment made of such material.
- To make waterproof or water-resistant.
- Unaffected by water.
- Made of or covered with material that doesn't allow water in.
- Incapable of failing; unassailable.