A klakluk was a Venusian animal that, according to the Third Doctor, resembled a "moose with no horns". They had two heads, which were useful for ensuring they couldn't be surprised by predators, but also made the beasts appear nervous, because they "didn't know whether they were coming or going."
An expression involving klakluk was, "You'd swallow a klakluk and choke on a Menian dust fly". It meant that some people could handle big things, but stumbled over minutiae. The Doctor used the expression to Sarah Jane Smith, who was having trouble accepting that he could travel back to Atlantis. He couldn't understand why she was sceptical of his ability to travel back to Atlantis, since she already knew he could travel back to Irongron's castle in the Middle Ages. (AUDIO: The Paradise of Death)