"Who cares?” (2,4)
I believe the answer is:
so what
'who cares?' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for so what that I've seen before include "It's unimportant, right?" , "It's unimportant, who cares" , "Does it make any difference" , "Does it matter?" , "Expresses contempt or lack of interest" .)