A leading lady and good Scots queen (6)

I believe the answer is:

guider

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

'a leading' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'lady and good scots queen' is the wordplay.
'lady' becomes 'gui'.
'and' is a charade indicator (letters next to each other).
'good scots queen' becomes 'der' (I am not sure about this - if you are sure you should believe this answer much more).
'gui'+'der'='GUIDER'

Can you help me to learn more?

(Another definition for guider that I've seen is " group leader".)

I've seen this clue in the Evening Standard.

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