Old Republican is supported by prayer (6)
I believe the answer is:
orison
'prayer' is the definition.
('orison' can be a synonym of 'prayer')
'old republican is supported by' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'republican' becomes 'r' (abbreviation).
'supported by' becomes 'on'.
'o'+'r'+'is'+'on'='ORISON'
(Other definitions for orison that I've seen before include "Prayer - O no sir! (anag)" , "O, no, sir, it makes a prayer" , "Supplication" , "An archaic or literary prayer" .)