Wants to sound negligent (5)
I believe the answer is:
'wants' is the definition.
('lack' can be a synonym of 'want')
'sound negligent' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'negligent' becomes 'lax' (similar in meaning).
'lax' sounds like 'LACKS'.
'to' acts as a link.
(Other definitions for lacks that I've seen before include "Needs" , "Is without, wants" , "Wants, is deficient in" , "Doesn't have" , "Has a want" .)