A trap's reset in an ancient city (6)
I believe the answer is:
sparta
'ancient city' is the definition.
(I've seen this before)
'a trap's reset in' is the wordplay.
'reset' is an anagram indicator.
'in' means one lot of letters goes inside another.
'traps' with letters rearranged gives 'sprta'.
'a' put inside 'sprta' is 'SPARTA'.
'an' is the link.
(Other definitions for sparta that I've seen before include "Ancient Greek city-state" , "Ancient Greek city state" , "in ancient Greece" , "Where to find ancient Greek" , "Ancient Greek city whose citizens were noted for hardiness" .)