Group of stars tackling saint's prayer (6)
I believe the answer is:
orison
'prayer' is the definition.
('orison' can be a synonym of 'prayer')
'group of stars tackling saint's' is the wordplay.
'group of stars' becomes 'orion' (I've seen this before).
'tackling' is an insertion indicator (I've seen this in other clues).
'saint' becomes 's'.
'orion' enclosing 's' is 'ORISON'.
(Other definitions for orison that I've seen before include "O, no, sir, it makes a prayer" , "Prayer - O no sir! (anag)" , "Supplication" , "An archaic or literary prayer" .)