Flexible thought process is adding something (15)

I believe the answer is:

supplementation

'adding something' is the definition.
(process of supplementing)

'flexible thought process' is the wordplay.
'flexible' becomes 'supple' (similar in meaning).
'thought process' becomes 'mentation' (formal term for use of the mind).
'supple'+'mentation'='SUPPLEMENTATION'

'is' is the link.

I've seen this clue in The Guardian.

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