The most common and most likely answer to this crossword clue is the 8 letter word ANNEALED
I've seen this crossword clue in The Daily Telegraph
A
N
N
E
A
L
E
D
Buddy explains!
The straight part
We know that "Hardened" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hardened = annealed.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 ANNEALED mean?
ANNEALED verb- To subject to great heat and then (often slow) cooling, and sometimes reheating and further cooling, for the purpose of rendering less brittle; to temper; to toughen.
- To cool glass slowly, to minimize internal stress.
- To burn colors onto a glass or other surface.
- To make a double-stranded nucleic acid by pairing a single strand with a complementary strand.
- To strengthen or harden.