Investigate cardinal in Roman Catholic church (5)
I believe the answer is:
recce
'investigate' is the definition.
(short for reconnoitre, to carry out reconnaissance)
'cardinal in roman catholic church' is the wordplay.
'cardinal' becomes 'E' (East, a cardinal direction).
'in' means one lot of letters goes inside another.
'roman catholic' becomes 'rc' (abbreviation for Roman Catholic).
'church' becomes 'CE' (abbreviation for Church of England).
'e' inserted into 'rc' is 'rec'.
'rec'+'ce'='RECCE'
(Other definitions for recce that I've seen before include "Preliminary survey" , "Exploratory foray (mil. colloq.)" , "Military survey" , "Reconnaissance" , "Shufti" .)