Safe old city in Home Counties (4)
I believe the answer is:
sure
'safe' is the definition.
(thesaurus)
'old city in home counties' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'in' means one lot of letters goes inside another.
'home counties' becomes 'se'.
'ur' going into 'se' is 'SURE'.
(Other definitions for sure that I've seen before include "Undoubtedly true" , "Definite" , "certainly in the USA" , "of course" , "Convinced" .)