City on theme having taken abnormal position (7)
I believe the answer is:
ectopic
'having taken abnormal position' is the definition.
(medical term)
'city on theme' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'on' is a charade indicator (letters next to each other).
'theme' becomes 'topic' (synonyms).
'ec'+'topic'='ECTOPIC'
(Other definitions for ectopic that I've seen before include "In an abnormal position" , "unusual pregnancy" , "out of place" , "In an unusual situation" .)