Poor English grade after that subject is not taught (10)
I believe the answer is:
illiterate
'not taught' is the definition.
(not taught to read and write)
'poor english grade after that subject' is the wordplay.
'poor' becomes 'ill' (I've seen this before).
'english' becomes 'e' (abbreviation).
'grade' becomes 'rate' (synonyms).
'after' means one lot of letters go next to another.
'that subject' becomes 'it' ('it' can refer to a previously mentioned subject).
'e'+'rate'='erate'
'erate' put after 'it' is 'iterate'.
'ill'+'iterate'='ILLITERATE'
'is' acts as a link.
(Other definitions for illiterate that I've seen before include "uneducated person" , "non-reader" , "Unable to read and write" , "twenty seven?" , "one who can't read" .)