Roofless London theatre in the old city (6)
I believe the answer is:
delphi
'the old city' is the definition.
(I've seen this before)
'roofless london theatre' is the wordplay.
'roofless' means to remove the first letter.
'london theatre' becomes 'adelphi' (I've seen this before).
'adelphi' with its first letter taken away is 'DELPHI'.
'in' acts as a link.
(Other definitions for delphi that I've seen before include "Scene of Greek oracle" , "'In Greek mythology, the site of the oracle (6)'" , "Ancient Greek sanctuary dedicated to Apollo" , "Place of Greek oracle" , "Dead ringer" .)