Extra foreign capital, exchanging Rupees and Marks (4)
I believe the answer is:
more
'extra' is the definition.
(I've seen this before)
'foreign capital exchanging rupees and marks' is the wordplay.
'foreign' indicates anagramming the letters (foreign order of the letters).
'capital' indicates taking the first letters.
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'marks' becomes 'mo' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letters of 'exchanging rupees' is 'er'.
'er' anagrammed gives 're'.
're' put after 'mo' is 'MORE'.
Can you help me to learn more?
(Other definitions for more that I've seen before include "See 9" , "victim of Henry" , "A greater amount" , "Extra (for Sir Thomas?)" , "To a greater extent" .)