Last of intrepid explorers vacated ancient city's ruins (8)
I believe the answer is:
destroys
'ruins' is the definition.
(to destroy is to ruin or break)
'last of intrepid explorers vacated ancient city's' is the wordplay.
'last of' indicates one should take the final letters.
'vacated' suggests removing the centre.
'ancient city's' becomes 'Troy's' (Troy was an ancient city).
'explorers' with its middle removed is 'es'.
The last letter of 'intrepid' is 'd'.
'd'+'es'+'troys'='DESTROYS'
(Other definitions for destroys that I've seen before include "Ruins or bankrupts" , "Puts down" , "undoing" , "Spoils completely, perhaps kills" , "Demolishes, ruins" .)