Capital fellow supporting end to casual working (6)
I believe the answer is:
london
'capital' is the definition.
(London is an example)
'fellow supporting end to casual working' is the wordplay.
'fellow' becomes 'don' (I've seen this before).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'end to' suggests the final letters.
'working' becomes 'on'.
The last letter of 'casual' is 'l'.
'l'+'on'='lon'
'don' after 'lon' is 'LONDON'.
(Other definitions for london that I've seen before include "England's capital city" , "UK's capital" , "City for the non-old, oddly" , "Where Boris Johnson might be" , "City of Piccadilly" .)