Hi. Replying to my own issue.
I've seen no apparent reason why clamd would work outside of systemd. However I have noticed that after I reboot Clearos server, its average load hits 10+. Observing things further, I've seen that systemd keeps restarting the service - I see clamd proces changing pid.
My best guess is that the machine is just too busy starting all the services it has to restart clamd several times. I see a lot of cpu usage goes to clamscan after boot. This goes on for a few minutes then settles - afterwards the clamd service starts and everything works.

It seems I will need to replace my hardware sometime soon.