Stops son being adopted by believers (7)
I believe the answer is:
desists
'stops' is the definition.
(I've seen this before)
'son being adopted by believers' is the wordplay.
'son' becomes 's'.
'being adopted by' indicates putting letters inside (I've seen this in other clues).
'believers' becomes 'deists' (people who believe in a god).
's' placed into 'deists' is 'DESISTS'.
(Other definitions for desists that I've seen before include "Ceases" , "Stops, refrains from" .)