She must step in line to succeed on stage (6,4)

'stage' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'she must step in line to succeed' is the wordplay.
I cannot really understand how this works, but
'line' could be 'chorus' (chorus line is a kind of line) and 'chorus' is located in the answer.
The remaining letters 'girl' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'on' is the link.

Can you help me to learn more?