Crossword Genius

What bees do when really busy? (4,6,3)

Ross

I believe the answer is:

make things hum

I'm a little stuck... Click here to teach me more about this clue!

'really busy?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'what bees do' is the wordplay.
I cannot quite see how this works, but
'do' could be 'make' (synonyms) and 'make' is present in the answer.
This explanation may well be incorrect...

'when' is the link.

Can you help me to learn more?

I've seen this clue in The Irish News and The Globe and Mail.
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