Alternator clutch???

Took car for mot on monday and tester advised that I should get the "cluth on the altenator" changed as it is making a noise.

I have heared it coming from the engine bay but don't know what/where it was coming from.

Is there such thing as a alternator clutch and where is it? how much does it cost to change?

Any help appreciated!:think:
 
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