Complex sentence: The group holds that the quality of "Slack" is of utmost importance--it is never clearly defined, but attaining it involves the avoidance of hard work and the embracing of leisure.
Split into simpler sentences: The group holds that the quality of "Slack" is of utmost importance--it is never clearly defined. Some adherents hold that attaining Slack involves the avoidance of hard work and the embracing of leisure.