Completely European and extremely popular church (5,4)
I believe the answer is:
every inch
'completely' is the definition.
(synonyms)
'european and extremely popular church' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'and' means one lot of letters go next to another.
'extremely' becomes 'very' (very can mean extremely or considerably).
'popular church' becomes 'inch' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'e'+'very'+'inch'='EVERY INCH'
Can you help me to learn more?
(Other definitions for every inch that I've seen before include "In all departments" , "Absolutely!" , "Entirely" , "Thoroughly" , "completely" .)