Clue: Open up a Conservative party – leadership of Boris is revoked (10)
Answer: ACCELERATE
Explanation: Definition: Open up
 A + C(abbrev. for the Conservative Party or a member of it) + "celebrate"(to acknowledge by holding a party, as was done by Boris in Downing Street) **minus ****1st letter of**(leadership of … is revoked) "Boris".
Defn: …/to widen the engine throttle of a motor vehicle, and thus, picking up speed.