An alarm clock discard is the discard of an illogical honor card in a suit held by declarer that asks partner who is on lead in the middle game to wake up and cash tricks in a side suit. It is essentially a command to look elsewhere for tricks. Here is an example.
My RHO became declarer in THREE NOTRUMP after the following auction.
RHO ONE DIAMOND, TWO SPADES by yours truly, DOUBLE (negative) by LHO, TWO NOTRUMP by RHO , THREE NOTRUMP by LHO.
I led the King of Spades, which declarer won, He played a club to the Ace (wish trick, deuce, trey, four). When he played a club up, he finessed the nine. I pitched a diamond, Declarer cashes three rounds of diamonds. They break. He plays a SPADE, I win, and now I have to cash out for one down. I am staring at Q763 on my left, and I have KJx. Unbeknownst to me partner has AT9x.
We can cash 4 hearts. How do I know to do this? Simple. Partner should pitch the CLUB QUEEN, and alarm clock signal. This cannot mean interest in clubs, since declarer is marked with the club K (he started with K9x). Now I should switch with confidence to HK, then HJ and a third heart to take 4 fast heart tricks.