Capital fellow supporting end to casual working (6)
I believe the answer is:
Here is my best explanation:
'capital' is the definition.
(London is an example)
'fellow supporting end to casual working' is the subsidiary indication.
'fellow' becomes 'don' (I've seen this before).
'supporting' means one lot of letters go next to another.
'end to' indicates one should take the final letters.
'working' becomes 'on'.
The final letter of 'casual' is 'l'.
'don' put after 'lon' is 'london'.
(Other definitions for london that I've seen before include "Where Boris Johnson might be", "England's capital city", "English capital", "Jack - , US author (White Fang); city", "UK's capital", "The Cockneys' city", "UK Capital city", "City of Piccadilly", "UK capital" and "source of this paper".)