Dear Jackie,

according to the OED, clock was a general term for beetle used in the north of England - origin unknown - often accompanied with a further descriptor. Clock-bee for instance was a flying beetle, clock-a-clay or clock-leddy was a ladybird ( I think you call it a ladybug in the USA). I don't know how far back rhyming slang was first used, but your question brought this to mind, crossing with the children's chants thread that was going a few weeks ago:

Ladybird, ladybird fly away home,
Your house is on fire and your children all gone,
Except for one, her name is Nan,
And she is hiding under the frying pan.

Children used to recite this nonsense rhyme when they found a ladybird.

dxb