A prayer (6)
I believe the answer is:
orison
'a prayer' is the definition.
('orison' can be a synonym of 'prayer')
This is the entire clue.
(Other definitions for orison that I've seen before include "An archaic or literary prayer" , "Supplication" , "Prayer - O no sir! (anag)" , "O, no, sir, it makes a prayer" .)