Southern fellow restraining endlessly caustic church worker (9)
I believe the answer is:
sacristan
'church worker' is the definition.
(I've seen this before)
'southern fellow restraining endlessly caustic' is the wordplay.
'southern' becomes 'S' (abbreviation).
'fellow' becomes 'stan' ().
'restraining' indicates putting letters inside.
'endlessly' means to remove the last letter.
'caustic' becomes 'acrid' (I've seen this before).
's'+'stan'='sstan'
'acrid' with its last letter taken away is 'acri'.
'sstan' enclosing 'acri' is 'SACRISTAN'.
(Other definitions for sacristan that I've seen before include "Church official" , "Church officer in charge of sacred vessels etc" , "Sexton" , "Church worker" , "Church officer in charge of sacred objects" .)