A German turning back to welcome city relative (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'a german turning back to welcome city' is the wordplay.
'a german' becomes 'ein' ('a' in German).
'turning back' says the letters should be written in reverse.
'to welcome' is an insertion indicator (in sense of 'inviting in').
'city' becomes 'ec' (the City of London postcode).
'ein' back-to-front is 'nie'.
'nie' going around 'ec' is 'NIECE'.
(Other definitions for niece that I've seen before include "Daughter of a sibling" , "Family girl" , "Kinswoman" , "Brother's girl" , "girl further down the line" .)