Didn't participate as a stand-in (3,3)

I believe the answer is:

sat out

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'didn't participate' is the definition.
(I've seen this in another clue)

'a stand-in' is the wordplay.
'stand' becomes 'stout' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'a' inserted within 'stout' is 'SAT OUT'.

'as' is the link.

Can you help me to learn more?

(Another definition for sat out that I've seen is " stayed till the end".)

I've seen this clue in The Irish Independent.

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