Machinist's function (8)

I believe the answer is:

operator

This clue is a double definition.

'machinist's' is the first definition.
(I've seen this before)

'function' is the second definition.
(operator is a kind of function)

(Other definitions for operator that I've seen before include "Person at switchboard, etc" , "Speculator" , "Controller, of telephone say" , "Agent that runs some apparatus or machine" , "Mechanic" .)

I've seen this clue in The Telegraph.

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