Bosses once associated with City of London (4)
I believe the answer is:
exec
'london' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'bosses once associated with city' is the wordplay.
'bosses once' becomes 'ex' (I can't justify this - if you can you should believe this answer much more).
'associated with' means one lot of letters go next to another.
'city' becomes 'ec' (the City of London postcode).
'ex'+'ec'='EXEC'
'of' is the link.
Can you help me to learn more?
(Other definitions for exec that I've seen before include "big cheese" , "CEO?" , "Board member" , "suit" , "Senior manager (abbr)" .)