In seconds a dreadful lack of harmony (10)
I believe the answer is:
dissonance
'lack of harmony' is the definition.
(I've seen this before)
'in seconds a dreadful' is the wordplay.
'dreadful' indicates anagramming the letters.
'in'+'seconds'+'a'='insecondsa'
'insecondsa' is an anagram of 'DISSONANCE'.
(Other definitions for dissonance that I've seen before include "Harsh sound" , "quartet?" , "Lack of harmony" , "Discordance" , "sounds harsh" .)