Making obscure knowledge challenging to acquire (9)

I believe the answer is:

darkening

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'making obscure' is the definition.
The answer and definition can be both related to change as well as being present participle verbs.
Perhaps you can see an association between them that I don't see?

'knowledge challenging to acquire' is the wordplay.
'knowledge' becomes 'ken' (ken is a kind of knowledge).
'challenging' becomes 'daring' (daring is a kind of challenging).
'to acquire' indicates putting letters inside.
'ken' inserted within 'daring' is 'DARKENING'.

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(Another definition for darkening that I've seen is " becoming more sombre".)

I've seen this clue in The Independent.

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