Sharpest lens maker employed this grinding tool (6)

I believe the answer is:

pestle

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

'grinding tool' is the definition.
(I've seen this before)

'sharpest lens maker employed' is the wordplay.
'PESTLE' is hidden within the letters.
But, I am unsure how this is indicated.

'this' acts as a link.

Can you help me to learn more?

(Other definitions for pestle that I've seen before include "Partner to the mortar" , "Small club for use with mortar" , "Mortar-grinding tool" , "Grinder for mortar" , "Small club, goes with mortar" .)

I've seen this clue in The Irish Times.

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