Mixture of old and new working capital (6)
I believe the answer is:
london
'capital' is the definition.
(capital of the United Kingdom)
'mixture of old and new working' is the wordplay.
'mixture of' is an anagram indicator (I've seen this in other clues).
'and' means one lot of letters go next to another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'working' becomes 'on' (on can mean working or activated).
'old' after 'n' is 'nold'.
'nold' with letters rearranged gives 'lond'.
'lond'+'on'='LONDON'
(Other definitions for london that I've seen before include "City for the odd non-old" , "- Clay; - pride; - plane" , "UK Capital city" , "English capital" , "City of the Cockneys" .)