German city engineer's inner nature (7)
I believe the answer is:
essence
'inner nature' is the definition.
(I've seen this before)
'german city engineer's' is the wordplay.
'german city' becomes 'essen' (city in Germany).
'engineer' becomes 'ce'.
'essen'+'ce'='ESSENCE'
(Other definitions for essence that I've seen before include "quality of perfume?" , "Core, most vital part" , "The heart of" , "Basic quality" , "Perfume or concentrated extract" .)