Being slack, is said to be one of the 'have nots' (3)
I believe the answer is:
lax
'being slack' is the definition.
('lax' can be a synonym of 'slack')
'said to be one of the have nots' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
The remaining letters 'lx' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for lax that I've seen before include "Slapdash" , "Not sufficiently strict" , "Insufficiently strict" , "Loose, careless" , "without tension" .)