Crossword Genius

Slack? If not interested, you couldn't ___ (8)

Ross

I believe the answer is:

careless

'slack?' is the definition.
(synonyms)

'not interested you couldn't' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'less' (I've seen this in other clues) and 'less' is located in the answer.
The remaining letters 'care' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'if' acts as a link.

(Other definitions for careless that I've seen before include "Marked by lack of attention" , "Not paying enough attention" , "Lax" , "Negligent - nonchalant" , "Inattentive to one's work" .)

I've seen this clue in The Guardian.
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