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