Lack of interest or enthusiasm (6)
I believe the answer is:
apathy
'lack of interest or enthusiasm' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for apathy that I've seen before include "Detachment" , "Lack of interest or enthusiasm" , "Disinclination to shift" , "Indifference - listlessness" , "Absense of interest or emotion" .)