Argue when against following school subject (6)
I believe the answer is:
reason
'argue' is the definition.
(synonyms)
'when against following school subject' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'against' becomes 'on' (I've seen this in other clues).
'following' is a charade indicator (letters next to each other).
'school subject' becomes 'RE' (Religious Education).
'as'+'on'='ason'
'ason' after 're' is 'REASON'.
(Other definitions for reason that I've seen before include "Justification; sanity" , "Cause; mind's power" , "Power of thought" , "Employ logic" , "Cause - logic" .)