Apathy turning to defiance if son replaces daughter (9)

I believe the answer is:

indolence

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'apathy' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for indolence that I've seen before include "Lack of activity" , "Laziness" , "Inactivity resulting from dislike of work" , "sloth" , "doing very little" .)

I've seen this clue in The Guardian.

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