Carelessness is clear in retrospect about son getting bad grade (9)
I believe the answer is:
disregard
'carelessness' is the definition.
(thesaurus)
'clear in retrospect about son getting bad grade' is the wordplay.
'clear' becomes 'rid' (synonyms).
'in retrospect' says the letters should be written in reverse.
'about' means one lot of letters goes inside another.
'son getting' becomes 's' (I've seen this in other clues).
'bad' is an anagram indicator.
'rid' backwards is 'dir'.
'dir' enclosing 's' is 'disr'.
'grade' with letters rearranged gives 'egard'.
'disr'+'egard'='DISREGARD'
'is' is the link.
(Other definitions for disregard that I've seen before include "Overlook - inattention" , "Push aside" , "Neglect, ignore" .)