Fog covering old, old city on one side of the bridge with rising water perhaps (8)
I believe the answer is:
moisture
'one side of the bridge with rising water perhaps' is the definition.
I can't judge whether this definition defines the answer.
'fog covering old old city' is the wordplay.
'fog' becomes 'mist' (mist is a kind of fog).
'covering' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'old city' becomes 'ure' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mist' enclosing 'o' is 'moist'.
'moist'+'ure'='MOISTURE'
'on' is the link.
Can you help me to learn more?
(Other definitions for moisture that I've seen before include "Small amount of liquid causing dampness" , "Small amount of liquid that causes dampness" , "Diffused liquid such as vapour or condensation" , "Liquid in small quantity, in the air say" , "Liquid in vapour or spray form" .)