Expert touring city in desert (6)
I believe the answer is:
defect
'desert' is the definition.
(I know that desert can be written as defect)
'expert touring city' is the wordplay.
'expert' becomes 'deft' (I've seen this in another clue).
'touring' means one lot of letters goes inside another (touring can mean going round).
'city' becomes 'EC' (postcode for the City of London).
'deft' going around 'ec' is 'DEFECT'.
'in' acts as a link.
(Other definitions for defect that I've seen before include "Lack" , "Blemish, fault" , "Abandon one's country in favour of an opposing one" , "Failing or imperfection" , "error" .)