Musically irregular concoction of harp and disco (9)
I believe the answer is:
rhapsodic
'musically irregular' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'concoction of harp and disco' is the wordplay.
'concoction of' indicates anagramming the letters (I've seen 'concoction' mean this).
'and' is a charade indicator (letters next to each other).
'harp'+'disco'='harpdisco'
'harpdisco' is an anagram of 'RHAPSODIC'.
Can you help me to learn more?
(Other definitions for rhapsodic that I've seen before include "Wildly emotional" , "of utter delight" , "Expressing ecstatic feeling" , "Like an irregular and emotional composition" , "Freely emotional" .)