Headgear for a pig! Who cares? (2,4)
I believe the answer is:
so what
'who cares?' is the definition.
(I've seen this before)
'headgear for a pig' is the wordplay.
'headgear' becomes 'hat' (hat is a kind of headgear).
'for' means one lot of letters go next to another (I've seen this in other clues).
'a pig' becomes 'sow' (I've seen this in another clue).
'hat' after 'sow' is 'SO WHAT?'.
(Other definitions for so what that I've seen before include "I don't care" , "Does it matter?" , "I couldn't care less" , "Big deal!" , "Expresses contempt or lack of interest" .)