Crossword Genius

Police officer, one demanding payment for engine booster (12)

Ross

I believe the answer is:

supercharger

'engine booster' is the definition.

'police officer one demanding payment' is the wordplay.
'police officer' becomes 'super' (term for a senior police officer).
'one demanding payment' becomes 'charger' (somebody who charges a fee).
'super'+'charger'='SUPERCHARGER'

'for' acts as a link.

(Other definitions for supercharger that I've seen before include "Engine efficiency improver" , "that makes for efficient transport" , "Device improving an engine's efficiency" .)

I've seen this clue in The Times.
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