The heart of a German city church (7)
I believe the answer is:
essence
'the heart of' is the definition.
(I've seen this before)
'a german city church' is the wordplay.
'a german city' becomes 'essen' (city in Germany).
'church' becomes 'CE' (abbreviation for Church of England).
'essen'+'ce'='ESSENCE'
(Other definitions for essence that I've seen before include "Extract; intrinsic nature" , "Soul" , "The most distinctive element" , "Cologne, perhaps?" , "required property" .)