Old work in attempt to support the belief system (9)
I believe the answer is:
theosophy
'belief system' is the definition.
(I've seen this before)
'old work in attempt to support the' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'work' becomes 'op' (abbreviation for opus).
'in' indicates putting letters inside.
'attempt to' becomes 'shy' (I've seen this in other clues).
'support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'op' placed into 'shy' is 'sophy'.
'o'+'sophy'='osophy'
'osophy' after 'the' is 'THEOSOPHY'.
(Other definitions for theosophy that I've seen before include "Form of religious belief" , "belief system" , "religious system" , "thought about religion" , "divine inspiration" .)