Capital city’s extremists fleeing industrial action (4)
I believe the answer is:
oslo
'capital city's' is the definition.
(capital of Norway)
'extremists fleeing industrial action' is the wordplay.
'extremists fleeing' indicates the central letters (letters at the extremes of the word are removed).
'industrial action' becomes 'go-slow' (when workers deliberately work slowly).
The central letters of 'goslow' are 'OSLO'.
(Other definitions for oslo that I've seen before include "Scandinavian city" , "Norwegian chief port" , "Nordic capital" , "Norway's capital city" , "Former Christiania" .)