Sorry for late reply. Probably you don’t need it anymore. Anyhow…
Other system logs are rotated using logrotate, but nextcloud.log has not been configured to use it. @greavette, is it worth?, could this be a new feature request for the @dev_team ?
Nextcloud has it’s own log rotation mechanism, handled by the log_rotate_size config parameter, which should serve your purpose.
I have tried it with the default Nethserver Nextcloud package, and the log was not rotated. 
IMO it would be better to rotate the logs rather than clear or delete them. But here are two ways to clear it:
> /var/lib/nethserver/nextcloud/nextcloud.log
or
truncate /var/lib/nethserver/nextcloud/nextcloud.log --size 0