Busy, cruel, talks are so dull (10)

I believe the answer is:

lacklustre

I'm a little stuck... Click here to teach me more about this clue!

'so dull' is the definition.
'lacklustre' can be an answer for 'dull' (similar in meaning). I'm not sure about the 'so' bit.

'busy cruel talks' is the wordplay.
'busy' indicates an anagram.
'cruel'+'talks'='crueltalks'
'crueltalks' anagrammed gives 'LACKLUSTRE'.

'are' acts as a link.

Can you help me to learn more?

(Other definitions for lacklustre that I've seen before include "Dull; boring" , "Unimaginative" , "Prosaic" , "Drab" , "Without vitality" .)

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

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