Mark firm losing half of capital (5)
I believe the answer is:
colon
'mark' is the definition.
(I know that colon is a type of punctuation)
'firm losing half of capital' is the wordplay.
'firm losing' becomes 'co' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'half of capital' becomes 'lon' (half of 'London').
'co'+'lon'='COLON'
Can you help me to learn more?
(Other definitions for colon that I've seen before include "cut short dependency?" , "unit of foreign currency" , "Panamanian port" , "Punctuate part of the large intestine" , "Part of the intestine - punctuation mark" .)