Possible Clues:

• Laconic reply to 25-Down, perhaps
• Reply to 'Whatcha doin'?'
• Response to 'Whatcha doin'?'
• "___ doin'!"
• '-- to it!' ('Simple!')
• '-- doin'!' ('Forget it!')
• Springsteen's '57 Channels (And ___ On)'
• 'I got ___'
• See 34A
• 'You ain't seen -- yet!'