Note: This is an excerpt from the Playback User Guide.
Use MIDI Clock to sync the tempo of your virtual instruments, external controllers, guitar delay pedals, and more with Playback. Send MIDI Clock data out of any selected MIDI port while Tracks are playing to keep any MIDI Clock-supported instruments and music equipment in time with Playback.
MIDI Clock is available with a Playback Premium subscription.
Select MIDI Ports to Send MIDI Clock
Navigate to the MIDI Settings tab, and scroll to the bottom of the page:
Tap on the "Select Ports" row, and select which available MIDI Ports you would like to send MIDI Clock to.
Once enabled, MIDI Clock will be sent to selected MIDI Ports anytime tracks are playing. The outgoing tempo will update in real time based on the BPM of the currently playing song or click track.