Ola Gerne,
MIDI is often regarded as "plug and play" for musicians (AKA dummies, SCNR). If at all, this may be true for those who only want to connect keyboards to keyboard extenders. In general however, MIDI is a nothing else as a plain computer-interface and You will have to read and learn fom Your elektron analog rytm's MIDI implementation.
I do not know that machine in special, but surfed a bit. You have pads and of course a MIDI out, which You already use to send MIDI clock/sync signals. Hopefully/probably Your machine will also be able to send pad-presses as MIDI keypress message via MIDI out and also to configure, which MIDI-key will be sent for a specific pad.
The trick is to make one of Your pads start the MIDI clock/sync
and send a MIDI keypress to P08 at the same time. You will loose that Pad for those "funny other things", but that's the trade...
- configure a pad to send the MIDI keypress needed via a MIDI channel You also set on P8
- configure that same pad to start the groove (= the MIDI clock)
This will make groove and P08 start at the same time by simply pressing that pad.
You will also need to control the sequence's/arpeggio's stop, which raises another problem:
Most drum machines don't really care about key release, as drummers normally also only hit their instruments, resulting in instrument-defined lengths of drum sounds. Some machines will send a key release directly after keypress, which in Your case would directly stop the arpeggio/sequence. Other machines may keep the key press for "some time", for as long as the pad is pressed, never send key releases, or may configure the related behaviour...
So You may have to configure Your machines behaviour, keep the pad or P08's sustain pedal pressed as long as You want P08 to sequence/arpeggiate. If Your machine is very flexible, it may even be able to send a "sustain pedal up" at certain conditions: Do whatever suits Your machine's features and possibilities.
HTH y saludos desde Aachen
Martin