No sir! Love back in prayer! (6)
I believe the answer is:
orison
'prayer' is the definition.
(synonyms)
'no sir love back' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'back' says the letters should be written in reverse.
'no'+'sir'+'o'='nosiro'
'nosiro' backwards is 'ORISON'.
'in' acts as a link.
(Other definitions for orison that I've seen before include "Prayer - O no sir! (anag)" , "An archaic or literary prayer" , "Supplication" , "O, no, sir, it makes a prayer" .)