Fool in defence, City's back (5)
I believe the answer is:
wally
'fool' is the definition.
(wally can mean a stupid person)
'defence city's back' is the wordplay.
'defence' becomes 'wall' (wall is a kind of defence).
'city's back' becomes 'y' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'wall'+'y'='WALLY'
'in' acts as a link.
(Other definitions for wally that I've seen before include "A fool (sl.)" , "Idiot (slang)" , "Complete idiot" .)