Crossword Genius

Fly into a rage when they express enthusiasm too boisterously? (2,5)

Ross

I believe the answer is:

go crazy

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'fly' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

I've seen this clue in The Irish Times.
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