Bit a ruler when joining in (9)

I believe the answer is:

partaking

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

'when joining in' is the definition.
I can't judge whether this definition defines the answer.

'bit a ruler' is the wordplay.
'bit' becomes 'part' (I've seen this before).
'ruler' becomes 'king' (king is a kind of ruler).
'part'+'a'+'king'='PARTAKING'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app