Apathy shown in a quarrel (12)
I believe the answer is:
indifference
'apathy' is the definition.
(I know that apathy can be written as indifference)
'in a quarrel' is the wordplay.
'a quarrel' becomes 'difference' (I've seen this before).
'in'+'difference'='INDIFFERENCE'
'shown' is the link.
(Other definitions for indifference that I've seen before include "Lack of involvement" , "Lack of concern" , "Unconcern" , "Lack of interest" .)