Write about ancient city's tranquillity (9)
I believe the answer is:
composure
'tranquillity' is the definition.
(I've seen this before)
'write about ancient city's' is the wordplay.
'write' becomes 'compose' ('compose' can be a synonym of 'write'**).
'about' is an insertion indicator.
'ancient city' becomes 'ur'.
'compose' going around 'ur' is 'COMPOSURE'.
(Other definitions for composure that I've seen before include "Aplomb" , "State of calmness" , "Calm state" , "Calm self-control" , "Self-possession" .)