Regular myth, rich are spoilt (8)
I believe the answer is:
rhythmic
'regular' is the definition.
(associated in meaning)
'myth rich are spoilt' is the wordplay.
'are spoilt' is an anagram indicator.
'myth'+'rich'='mythrich'
'mythrich' is an anagram of 'RHYTHMIC'.
(Other definitions for rhythmic that I've seen before include "Relating to musical pulse" , "Rich myth with regular beat" , "Throbbing" , "With a strong sound pattern" , "Recurring with measured regularity" .)