Just old city left inside (4)
I believe the answer is:
only
'just' is the definition.
(I know that just can be written as only)
'old city left inside' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'city' becomes 'ny' (short for New York).
'left' becomes 'l' (common abbreviation).
'inside' means one lot of letters goes inside another.
'ny' enclosing 'l' is 'nly'.
'o'+'nly'='ONLY'
(Other definitions for only that I've seen before include "Mere" , "Solely; except that" , "And no one else" , "Existing alone with no other, perhaps a child" , "Simply" .)