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