Lingerie shortened, not wanting zip that's slack (9)
I believe the answer is:
negligent
'slack' is the definition.
(similar in meaning)
'lingerie shortened not wanting zip' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'nt' (synonyms) and 'nt' is found within the answer.
The remaining letters 'neglige' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "Failing to take proper care" , "Not taking due care" , "Not minding" , "sloppy" , "Careless, remiss" .)