Stop believer having son (6)
I believe the answer is:
desist
'stop' is the definition.
(desist means to stop)
'believer having son' is the wordplay.
'believer' becomes 'deist' (person who believes in a god).
'having' is an insertion indicator.
'son' becomes 's' (genealogical abbreviation for son).
'deist' enclosing 's' is 'DESIST'.
(Other definitions for desist that I've seen before include "Forbear" , "Stop, abstain" , "Stop, cease activity" , "Refrain (from doing something)" , "Whoa, cease" .)