Couple of poor grades withheld by lady in disgrace (6)
I believe the answer is:
defame
'disgrace' is the definition.
(thesaurus)
'couple of poor grades withheld by lady' is the wordplay.
'couple of poor grades' becomes 'fam' (I am not sure about this - if you are sure you should believe this answer much more).
'withheld by' means one lot of letters goes inside another.
'lady' becomes 'Dee'.
'fam' placed into 'dee' is 'DEFAME'.
'in' is the link.
(Other definitions for defame that I've seen before include "libel" , "Damage the reputation (of someone)" , "Attack someone's good name" , "Injure the reputation of" , "to pretetrate slander?" .)