Half of capital includes a sum advanced (4)
I believe the answer is:
loan
'sum advanced' is the definition.
(sum of money advanced to someone)
'half of capital includes a' is the wordplay.
'half of capital' becomes 'lon' (half of 'London').
'includes' is an insertion indicator.
'lon' placed around 'a' is 'LOAN'.
(Other definitions for loan that I've seen before include "One acquired temporarily" , "It's given but has to be returned" , "Money borrowed; lend" , "It's given temporarily" , "Mortgage" .)