Joshua originally ruining heroic old city (7)
I believe the answer is:
jericho
'old city' is the definition.
'joshua originally ruining heroic' is the wordplay.
'originally' suggests taking the first letters.
'ruining' indicates anagramming the letters.
The initial letter of 'joshua' is 'j'.
'heroic' is an anagram of 'ericho'.
'j'+'ericho'='JERICHO'
(Other definitions for jericho that I've seen before include "West Bank city" , "City taken by Joshua" , "Rich Joe is found where Joshua fought the battle" , "Town that fell to Joshua" , "Biblical walled city" .)