Crossword Genius

Controller of number of vehicles you subsequently overheard (9)

Ross

I believe the answer is:

regulator

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'controller' is the definition.
(I know that regulator is a type of controller)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for regulator that I've seen before include "source of rules?" , "Controller" , "Ofsted perhaps" , "Such as OFGAS, OFTEL etc" , "Governor" .)

I've seen this clue in The Times.
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