Subject is Gable: Tortured Artist (7)
I believe the answer is:
algebra
'subject' is the definition.
(algebra is a kind of subject)
'gable tortured artist' is the wordplay.
'tortured' is an anagram indicator.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'gable' anagrammed gives 'algeb'.
'algeb'+'ra'='ALGEBRA'
'is' is the link.
(Other definitions for algebra that I've seen before include "school subject" , "part of curriculum" , "exercise in school?" , "A garble (anag) - branch of mathematics" , "Numbers and letters branch of maths" .)