The most common and most likely answer to this crossword clue is the 6 letter word HARDEN
I've seen this crossword clue in The Daily Telegraph
H
A
R
D
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More clues leading to the result HARDEN
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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 HARDEN mean?
HARDEN verb- To become hard (tough, resistant to pressure).
- To make something hard or harder (tough, resistant to pressure).
- To strengthen.
- To modify (a website or other system) to make it resistant to malicious attacks.
- To become or make (a person or thing) resistant or less sensitive.
- To become or make (a consonant) more fortis.
- A coarse kind of linen made from hurds.