Subject Gable posed by artist (7)
I believe the answer is:
algebra
'subject' is the definition.
(algebra is a kind of subject)
'gable posed by artist' is the wordplay.
'posed' indicates anagramming the letters (I've seen 'pose' mean this (the letters must be re-posed into a new order)).
'by' says to put letters next to each other.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'gable' with letters rearranged gives 'algeb'.
'algeb'+'ra'='ALGEBRA'
(Other definitions for algebra that I've seen before include "Branch of maths - real bag (anag)" , "Part of mathematics" , "Branch of generalised arithmetic" , "exercise in school?" , "letters here set out the problem" .)