Is untidy, causing lack of harmony (8)
I believe the answer is:
disunity
'lack of harmony' is the definition.
'disunity' can be an answer for 'lack' (I have seen 'Lack of agreement' mean 'disunity' so perhaps 'lack' could also mean 'disunity'). I am not certain of the 'of harmony' bit.
'is untidy causing' is the wordplay.
'causing' is an anagram indicator.
'is'+'untidy'='isuntidy'
'isuntidy' anagrammed gives 'DISUNITY'.
Can you help me to learn more?
(Other definitions for disunity that I've seen before include "Lack of oneness" , "Lack of solidarity" , "conflict" , "Lack of agreement" .)