Subject to terrible end in ancient city (5)
I believe the answer is:
under
'subject to' is the definition.
(being subject to a law is being under it)
'terrible end in ancient city' is the wordplay.
'terrible' indicates an anagram.
'in' indicates putting letters inside.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'end' with letters rearranged gives 'nde'.
'nde' inserted into 'ur' is 'UNDER'.
(Other definitions for under that I've seen before include "See 7" , "Being short" , "Inferior" , "Subject to the authority of" , "'Beneath, below (5)'" .)