Clue: Too bad if this little party screws up! (3,2,1,2)
Answer: BIT OF A DO
Explanation: Definition: party
***(too bad if)** [anag:*screws up*] 
*Not sure if my explanation of this clue works, but here goes – A "bit of a do" is an understatement for a large party, so it is "turned up" which somehow becomes "screwed up"? Hovis (comment 2) agrees with my parsing, but I'm not happy with the anagrind being so far from the fodder.