It measures speed comet interacts with Earth (10)

I believe the answer is:

tachometer

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'it measures speed comet interacts with earth' is the definition.
The definition suggests a singular noun which matches the answer.

This is all the clue.

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(Other definitions for tachometer that I've seen before include "rev counter" , "Instrument measuring speed of rotation" , "It measures revolutions" , "Teacher Tom (anag.)" .)

I've seen this clue in The Independent.

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