Broadcast Derby? Big deal! (2,4)
I believe the answer is:
so what
'big deal' is the definition.
(similar expressions of apathy)
'broadcast derby?' is the wordplay.
'broadcast' becomes 'sow' (sowing is a kind of broadcasting).
'derby?' becomes 'hat' (derby is a kind of hat).
'sow'+'hat'='SO WHAT?'
(Other definitions for so what that I've seen before include "Big deal!" , "Does it make any difference" , "It's unimportant, who cares" , "Expresses contempt or lack of interest" , "I'm not interested" .)