Irishmen detaining naughty boy — sauce's source? (3,5)

I believe the answer is:

soy beans

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'sauce's source?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'irishmen detaining naughty boy' is the wordplay.
'irishmen' becomes 'seans' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'detaining' means one lot of letters goes inside another.
'naughty' indicates an anagram.
'boy' is an anagram of 'oyb'.
'seans' placed around 'oyb' is 'SOY BEANS'.

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I've seen this clue in The Independent.

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