In the British musical HONK!  Ida sings "keep paddling, like the
clapper, just keep paddling with your flappers..."  What is the
clapper?

I'm going to go out on a limb here. I could find no discussion of this
online, so I'm offering three theories (saving the best one for last).

This might be referring to the clapper (also called a "crotalus") that
is sometimes used in Catholic ceremonies to summon people to the
services during periods such as Holy Week, when no bells were to be
rung. It's like a wooden rattle that you shake back and forth
continuously to make a clack-clack noise.

Another possibility: could the word be 'Klepper' rather than
'clapper'? Klepper is the trade name of a famous style of kayak.
Considering the reference to paddling, that makes some sense.

Klepper Folding Kayak
http://www.paddlin.com/kayak/klepper.html

But I think the best option is that this is a reference to a water
bird called the clapper rail, which paddles not only with its feet,
but also with its wings underwater. Clappers are also called "marsh
hens."

"As Audubon wrote of the Clapper Rail in his Birds of America (1842):

On the least appearance of danger, they lower the head, stretch out
the neck, and move off with incomparable speed, always in perfect
silence... They can readily submerge their normally buoyant bodies,
dive when pressed, and speed their paddling by using wings underwater.
So effectively do they maintain a low profile that their main nonhuman
predators are pike, black bass, and other predatory fish which feed on
their young."

Birds of Stanford: Thin as a Rail
http://www.stanfordalumni.org/birdsite/text/essays/Thin_as_a_Rail.html

I hope this helps! If anything is unclear, please request
clarification; I'll be glad to offer further assistance before you
rate my answer.

Best regards,
pinkfreud

Great to give several different explanations, even if some were
different words.  I would have liked the specific reference from the
show to be considered.