Lack of interest in a course with happy ending (6)
I believe the answer is:
apathy
'lack of interest' is the definition.
'a course with happy ending' is the wordplay.
'course' becomes 'path' ('path' can be a synonym of 'course'**).
'with' means one lot of letters go next to another.
'ending' suggests the final letters.
The last letter of 'happy' is 'y'.
'a'+'path'+'y'='APATHY'
'in' is the link.
(Other definitions for apathy that I've seen before include "Flatness" , "Indifferent quality" , "Absence of interest or emotion" , "Indifference - listlessness" , "Absense of interest or emotion" .)