Senior politician, one entering cathedral (8)
I believe the answer is:
minister
'senior politician' is the definition.
(eg cabinet minister)
'one entering cathedral' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' means one lot of letters goes inside another.
'cathedral' becomes 'minster' (minster is a kind of cathedral).
'i' inserted into 'minster' is 'MINISTER'.
(Other definitions for minister that I've seen before include "Servant of state or church" , "Merit sin (anag.)" , "One of the clergy" , "Senior politician" , "Tend" .)