### Clue: Prison heads want Tory politician to service 50% of vehicles (9)
### Answer: GOVERNORS 
 ### Explanation: Definition: Prison heads
 GOVE(Michael, a Tory politician) **plus**(to) RN(abbrev. for "Royal Navy", one of the military services in the UK) + **last 3 letters of**(50% of) "motors"(vehicles).