Regular myth, exploded, set in rich ground (8)
I believe the answer is:
rhythmic
'regular' is the definition.
('rhythmic' is associated in meaning with 'regular')
'myth exploded set in rich ground' is the wordplay.
'exploded' is an anagram indicator.
'set in' is an insertion indicator.
'ground' indicates an anagram (grind the letters ).
'myth' with letters rearranged gives 'ythm'.
'rich' anagrammed gives 'rhic'.
'ythm' inserted into 'rhic' is 'RHYTHMIC'.
(Other definitions for rhythmic that I've seen before include "Recurring with measured regularity" , "Throbbing" , "Having strong or regular beat as in music" , "With a strong sound pattern" , "Rich myth strangely recurs at regular intervals" .)