Some devout Catholics backed diplomacy (4)
I believe the answer is:
tact
'diplomacy' is the definition.
(tact can mean diplomacy or sensitivity)
'some devout catholics backed' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'backed' says the letters should be written backwards.
'tcat' is hidden in the letters of 'devout catholics'.
'tcat' back-to-front is 'TACT'.
(Other definitions for tact that I've seen before include "Consideration and sensitivity" , "Diplomatic skill" , "Diplomacy or discretion" , "Skill in dealing with people" , "Diplomatic sensitivity" .)