Uncertain events may produce heads! (4-3)

This clue might be a double definition.

'uncertain events' is the first definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.

'produce heads' is the second definition.
The definition and answer can be both types of natural phenomena as well as being plural nouns.
Maybe you can see an association between them that I can't see?

'may' is the link.

Can you help me to learn more?