Textile dealer is starting to experiment with a strengthening technique (9)

I believe the answer is:

mercerise

I'm a little stuck... Click here to teach me more about this clue!

'a strengthening technique' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'textile dealer is starting to experiment' is the wordplay.
'textile dealer' becomes 'mercer' (I've seen this before).
'starting to' indicates taking the first letters.
The initial letter of 'experiment' is 'e'.
'mercer'+'is'+'e'='MERCERISE'

'with' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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