Ancient city, show one that is knocked over (7)
I believe the answer is:
pompeii
'ancient city' is the definition.
(ancient Roman city)
'show one that is knocked over' is the wordplay.
'show' becomes 'pomp' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'knocked over' shows that the letters should be reversed in order.
'i'+'ie'='iie'
'iie' back-to-front is 'eii'.
'pomp'+'eii'='POMPEII'
(Other definitions for pompeii that I've seen before include "City destroyed but preserved by Vesuvius" , "Roman city destroyed in 79AD" , "Lava-buried Roman city" , "Ancient site" , "Town buried by Vesuvius 1st century a.d." .)