Capital letter's first seen by working man (6)
I believe the answer is:
london
'capital' is the definition.
(capital of the UK)
'letter's first seen by working man' is the wordplay.
'letter's first' becomes 'l' (dst letter of 'letter').
'seen by' means one lot of letters go next to another.
'working' becomes 'on' (on can mean working or activated).
'man' becomes 'don' (Don is an example).
'l'+'on'+'don'='LONDON'
(Other definitions for london that I've seen before include "- Clay; - pride; - plane" , "Home of Cockney" , "City for the non-old, oddly" , "English capital" , "British capital" .)