Old PM still working? He hasn't a b__ prayer (6)
I believe the answer is:
orison
'prayer' is the definition.
(I know that prayer can be written as orison)
'old pm still working? he hasn't a b' is the wordplay.
'old pm' becomes 'Boris' (Boris Johnson).
'still working?' becomes 'on' (switched on).
'he hasn't' is a deletion indicator.
'boris'+'on'='borison'
'borison' with 'b' taken away is '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" .)