Abscond from city in moist air (6)
I believe the answer is:
decamp
'abscond' is the definition.
('decamp' can be a synonym of 'abscond')
'city in moist air' is the wordplay.
'city' becomes 'ec' (the City of London postcode).
'in' means one lot of letters goes inside another.
'moist air' becomes 'damp' (I've seen this before).
'ec' placed within 'damp' is 'DECAMP'.
'from' is the link.
(Other definitions for decamp that I've seen before include "Depart suddenly or secretly" , "Depart quickly and in secret" , "Move on" , "Leave suddenly" , "Make sudden breakaway" .)