Farm animal with headgear is big deal (2,4)
I believe the answer is:
so what
'big deal' is the definition.
(similar expressions of apathy)
'farm animal with headgear' is the wordplay.
'farm animal' becomes 'sow' (female pig).
'with' says to put letters next to each other.
'headgear' becomes 'hat' (hats are a type of headgear).
'sow'+'hat'='SO WHAT?'
'is' acts as a link.
(Other definitions for so what that I've seen before include "It's unimportant, who cares" , "Big deal!" , "Does it matter?" , "I couldn't care less" , "It's unimportant, right?" .)