Stop believer welcoming Sabbath's beginning (6)
I believe the answer is:
desist
'stop' is the definition.
(desist means to stop)
'believer welcoming sabbath's beginning' is the wordplay.
'believer' becomes 'deist' (person who believes in a god).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'beginning' suggests taking the first letters.
The first letter of 'sabbaths' is 's'.
'deist' going around 's' is 'DESIST'.
(Other definitions for desist that I've seen before include "Stop, cease activity" , "'Refrain from, stop (6)'" , "Cease (from doing something)" , "Abstain, refrain" , "Stop (doing something)" .)