The most common and most likely answer to this crossword clue is the 9 letter word ANNEALING
I've seen this crossword clue in The Times
A
N
N
E
A
L
I
N
G
Buddy explains!
The straight part
We know that "toughening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case toughening = annealing.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 ANNEALING mean?
ANNEALING 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.
- The heating of solid metal or glass to high temperatures and cooling it slowly so that its particles arrange into a defined lattice.