A pastoral letter: some felt it useful (5)
I believe the answer is:
titus
'a pastoral letter' is the definition.
(Epistle to Titus, a pastoral epistle in the Bible)
'some felt it useful' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'TITUS' is hidden in the letters of 'felt it useful'.
(Other definitions for titus that I've seen before include "NT book" , "Roman emperor from AD79" , "Roman emperor with arch" , "& 20 Mervyn Peake character" , "good book" .)