Indifference in a way unknown (6)
I believe the answer is:
apathy
'indifference' is the definition.
(I know that indifference can be written as apathy)
'a way unknown' is the wordplay.
'way' becomes 'path' (path is a kind of way).
'unknown' becomes 'y' (algebra).
'a'+'path'+'y'='APATHY'
'in' acts as a link.
(Other definitions for apathy that I've seen before include "Absense of interest or emotion" , "Indifference, emotionlessness" , "Lack of excitement" , "lack of involvement" , "Disinclination to shift" .)