Lessing creates tension and suspense by using a sort of ticking clock in the climax of this story.  Jerry knows that he counted to one hundred and sixty while the older boys swam through the tunnel, so he knows that he has to be able to hold his breath for the same amount of time in order to accomplish the same feat.  As he swims through the tunnel, he keeps count in his head, "Fifty, fifty-one, fifty-two . . . . [...].  Seventy-one, seventy-two . . . ."  He soon reaches one hundred, and "His lungs were beginning to hurt."  Then, he reaches one hundred fifteen.  A few moments later, he counts one hundred fifteen again, but he knows he already counted to that.  "His head was swelling, his lungs cracking.  A hundred and fifteen, a hundred and fifteen pounded through his head, and he feebly clutched at rocks in the dark [...].  He felt he was dying."  When Jerry gets stuck on one hundred fifteen, he loses track of how long he's truly been underwater, and we have no idea how much further he can make it (nor does he know).  This is the moment of the most tension in the story because the clock is still ticking, so to speak, but we have no idea how much time has passed.  This is, literally, life or death for Jerry, and so the tension and anxiety are terrible.

What device does Lessing use to create a "ticking clock" effect in the story?

What Lessing does in lieu of having a ticking clock is to use Jerry's counting.  Jerry keeps counting off the seconds in various things and that acts just like a ticking clock.

The first time we see this is when all the boys dive in to the water and disappear.  Jerry starts counting the seconds.  At the fifty second mark he is terrified.  By the time he counts to one hundred, he starts to wonder if he should call for help.  He starts counting faster and faster (this increases our suspense more) but the boys finally come up at 160.

Jerry's counting is very much like a ticking clock and I think it performs the same function.