Old Nick initially ignored prayer (6)
I believe the answer is:
orison
'prayer' is the definition.
('orison' can be a synonym of 'prayer')
'old nick initially ignored' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'nick' becomes 'prison' (British slang).
'initially ignored' means to remove the first letter.
'prison' with its initial letter removed is 'rison'.
'o'+'rison'='ORISON'
(Other definitions for orison that I've seen before include "O, no, sir, it makes a prayer" , "Supplication" , "Prayer - O no sir! (anag)" , "An archaic or literary prayer" .)