Tells users improvement is rather dull (10)

I believe the answer is:

lustreless

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'dull' is the definition.
(similar in meaning)

'tells users improvement is rather' is the wordplay.
'tells users' anagrams to 'LUSTRELESS'.
Though, I'm not clear how this is indicated.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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