Tough hack entering empty city (5)
I believe the answer is:
chewy
'tough' is the definition.
(similar in meaning)
'hack entering empty city' is the wordplay.
'hack' becomes 'hew' (I've seen this before).
'entering' is an insertion indicator.
'empty' suggests removing the centre.
'city' with its centre removed is 'cy'.
'hew' put inside 'cy' is 'CHEWY'.
(Other definitions for chewy that I've seen before include "Soft and sticky (to eat)" , "Taking much mastication" , "With a texture like toffee" , "Tough to masticate" , "Requiring mastication, like toffee say" .)