Negligent missing son’s absence (4)
I believe the answer is:
lack
'absence' is the definition.
(thesaurus)
'negligent missing son's' is the wordplay.
'negligent' becomes 'slack' ('slack' can be similar in meaning to 'negligent').
'missing' indicates named letters should be taken away.
'slack' with 's' taken out is 'LACK'.
(Other definitions for lack that I've seen before include "Be without or short of" , "Be wanting or without" , "Absence, deficiency" , "Deficiency (of something)" , "Dearth" .)