Sources of satisfaction certainly involved in requests (9)

I believe the answer is:

pleasures

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'sources of satisfaction' is the definition.
The definition and answer can be both related to thinking as well as being plural nouns.
Maybe there's a link between them I don't understand?

'certainly involved in requests' is the wordplay.
'certainly' becomes 'sure' (synonyms).
'involved in' is an insertion indicator.
'requests' becomes 'pleas' (plea is a kind of request).
'sure' put into 'pleas' is 'PLEASURES'.

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I've seen this clue in The Independent.

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