Crossword Genius

Nil mud particles found on road-repairing tools (9,6)

Ross

I believe the answer is:

pneumatic drills

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'tools' is the definition.
(pneumatic drill is a kind of tool)

'nil mud particles found on road-repairing' is the wordplay.
'nil mud particles' can be anagrammed to 'PNEUMATIC DRILLS'.
But, I'm not clear how this is indicated.

Can you help me to learn more?

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