Crossword Genius

Function of Civil Service stopped by nothing (3)

Ross

I believe the answer is:

cos

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'function' is the definition.
(mathematical function)

I don't understand the remainder of the clue.

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(Other definitions for cos that I've seen before include "Function briefly" , "Greek island (alternative spelling): Trig abbrev!" , "Senior officers" , "Salad plant" , "naturally having longer leaves than most" .)

I've seen this clue in The Sunday Times and The Times.
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