Although it’s not possible by default to automate Ableton’s tempo to a specific clip using clip envelope automation, there is a Max for Live device called TEMPO that makes it possible. TEMPO allows you to directly change Ableton Live’s master tempo to the desired BPM speed. You can download and drag the TEMPO device onto a track. Choose a clip on that track, and automate TEMPO’s parameters inside that clip using Clip Envelope Automation.
Download the TEMPO Max for Live device at this link: