It can be stopped by request (3)

I believe the answer is:

bus

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'it can be stopped by request' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

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(Other definitions for bus that I've seen before include "Motor-coach" , "Charabanc" , "Something caught" , "Public service vehicle" , "People carrier" .)

I've seen this clue in the Evening Standard.

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