There is no manual clutch control exposed to users. For completeness this should be available via blueprint in the same way that the settering, throttle and brake are user controlled.
Open a QAGame level with a vehicle. i.e. TM-VehiclesCommon
Examine the vehicle blueprint. There is no clutch control exposed to users.
The vehicle movement component has user control over steering, throttle, brake, via SetBrake, SetThrottle, however there is not an equivalent for a clutch.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-181298 in the post.