That's the thing about poor grades, son (2,3)
I believe the answer is:
id est
'that's' is the definition.
Both the definition and answer are adverbs.
Maybe they are linked in a way I don't understand?
'thing about poor grades son' is the wordplay.
'thing' becomes 'it'.
'about' means one lot of letters goes inside another.
'poor grades' becomes 'de' (I've seen this in other clues).
'son' becomes 's' (genealogical abbreviation for son).
'de'+'s'='des'
'it' placed around 'des' is 'ID EST'.
'the' is the link.
Can you help me to learn more?
(Other definitions for id est that I've seen before include "I mean to say!" , "that is in Rome" , "Ie, in full" , "Namely" , "That is to say (Latin)" .)