Crossword Genius

Each girl has to be cast in supporting part (5,3)

Ross

I believe the answer is:

chair leg

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

'each girl has to be cast in supporting' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'part' is the wordplay.
I cannot quite understand how this works, but
'part' could be 'hair' (hair is a kind of part) and 'hair' is found within the answer.
The remaining letters 'cleg' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

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