Crossword Genius

Group encouraged to break a leg and what a member may need if successful (4)

Ross

I believe the answer is:

cast

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

'group' is the definition.
The answer is an example.

'encouraged to break a leg and what a member may need if successful' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
The remaining letters 'cst' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for cast that I've seen before include "Throw off; select (actor)" , "Allot roles in a play" , "Complement of actors in a play" , "Group of actors in a play" , "Form < > company" .)

I've seen this clue in The Independent.
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