Endeavour to surround old city (4)
I believe the answer is:
troy
'city' is the definition.
(Troy is an example)
'endeavour to surround old' is the wordplay.
'endeavour' becomes 'try' (synonyms).
'to surround' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'try' enclosing 'o' is 'TROY'.
(Other definitions for troy that I've seen before include "Besieged city which fell to Wooden Horse" , "Weight system - old city" , "Helen of --, beauty" , "Ancient city of the Iliad" , "See 7" .)