Take off from city in grip of wetness (6)
I believe the answer is:
decamp
'take off' is the definition.
(thesaurus)
'city in grip of wetness' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'in grip of' is an insertion indicator.
'wetness' becomes 'damp' (damp is a kind of wetness).
'ec' inserted within 'damp' is 'DECAMP'.
'from' is the link.
(Other definitions for decamp that I've seen before include "beat it" , "Fold tents and go" , "Mizzle" , "Go away suddenly or secretly - leaving tent?" , "Fold up tent and depart" .)