Attempt to besiege old city of Paris (4)
I believe the answer is:
troy
'city of paris' is the definition.
(Paris was a hero from Troy in mythology)
'attempt to besiege old' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'besiege' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'try' going around 'o' is 'TROY'.
(Other definitions for troy that I've seen before include "City besieged by Greeks to rescue Helen" , "Ancient city besieged to free beautiful Helen" , "Ancient city in Asia Minor" , "Wooden horse siege city" , "part of Asia Minor" .)