Schoolmate not about could become abhorrent (9)
I believe the answer is:
loathsome
'abhorrent' is the definition.
(I've seen this before)
'schoolmate not about could become' is the wordplay.
'not' is a deletion indicator.
'about' becomes 'c' (abbreviation for 'circa').
'could become' indicates anagramming the letters.
'schoolmate' with 'c' taken away is 'shoolmate'.
'shoolmate' is an anagram of 'LOATHSOME'.
(Other definitions for loathsome that I've seen before include "Hateful" , "gross" , "15 [ABHORRENT]" , "Horrid" , "Disgusting, nauseating" .)