Lack of harmony is issue in waltz, say (10)
I believe the answer is:
dissonance
'lack of harmony' is the definition.
(I've seen this before)
'is issue in waltz say' is the wordplay.
'issue' becomes 'son' (issue can mean someone's offspring).
'in' means one lot of letters goes inside another.
'waltz say' becomes 'dance' (waltz is a kind of dance).
'is'+'son'='isson'
'isson' placed within 'dance' is 'DISSONANCE'.
(Other definitions for dissonance that I've seen before include "Lack of harmony" , "Harsh sound" , "sounds harsh" , "Discordance" , "quartet?" .)