Test side with advantage (6-4)

I believe the answer is:

eleven-plus

'test' is the definition.
(exam taken by some UK school pupils)

'side with advantage' is the wordplay.
'side' becomes 'eleven' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'advantage' becomes 'plus' (I've seen this before).
'eleven'+'plus'='ELEVEN-PLUS'

(Another definition for eleven-plus that I've seen is " School exam".)

I've seen this clue in The Times.

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