Too old for leapfrog, we retreated away inside (4,3,2)

I believe the answer is:

grew out of

'too old for' is the definition.
(growing out of something is getting too old for it)

'leapfrog we retreated away inside' is the wordplay.
I cannot really understand how this works, but
'away' could be 'out' (similar in meaning) and 'out' is found in the answer.
This explanation may well be incorrect...

I've seen this clue in The Guardian.

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