### Clue: Regular myth set in rich ground (8)
### Answer: RHYTHMIC 
 ### Explanation: Definition: Regular
An envelope ('in') of YTHM, an anagram ('set') of 'myth' in RHIC, an anagram ('ground') of 'rich'. Perhaps 'ground' might better be regarded as the anagrind for both parts, and 'set in' as the envelope indicator.