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 in another clue).
'entering' indicates putting letters inside.
'empty' means to remove the middle letters.
'city' with its centre removed is 'cy'.
'hew' going into '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" .)