Familiar footwear, that is, in the city (7)
I believe the answer is:
wellies
'familiar footwear that is in the city' is the definition.
Although both the answer and 'familiar' are nouns, they are not in the same form of the noun.
'familiar footwear that is in the city' is the wordplay.
'familiar footwear' becomes 'ie' (I can't explain this - if you can you should believe this answer much more).
'that is in the' indicates putting letters inside.
'city' becomes 'wells' (I've seen this before).
'ie' put into 'wells' is 'WELLIES'.
Can you help me to learn more?
(Other definitions for wellies that I've seen before include "Muddy footwear?" , "Rubber boots (colloq.)" , "provincial students in the main" .)