Lack of enthusiasm (6)
I believe the answer is:
apathy
'lack of enthusiasm' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for apathy that I've seen before include "Complete lack of interest, indifference" , "Indifferent quality" , "Indifference - listlessness" , "lack of involvement" , "Indifference, emotionlessness" .)