Church helper’s fateful day: months in hospital (8)
I believe the answer is:
sidesman
'church helper's' is the definition.
'fateful day months in hospital' is the wordplay.
'fateful day' becomes 'ides' (Julius Caesar was killed on the ides of March).
'months' becomes 'm'.
'in' indicates putting letters inside.
'hospital' becomes 'san' (I've seen this in other clues).
'ides'+'m'='idesm'
'idesm' placed into 'san' is 'SIDESMAN'.
(Other definitions for sidesman that I've seen before include "Church officer" , "Churchwarden's assistant -- semi-sand (anag)" , "Church worker" , "Church official" .)