Son splits modest prize money (5)
I believe the answer is:
purse
'prize money' is the definition.
(I've seen this before)
'son splits modest' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'splits' indicates putting letters inside (inserted letters split open the word).
'modest' becomes 'pure' (I can't justify this - if you can you should give a lot more credence to this answer).
's' put into 'pure' is 'PURSE'.
Can you help me to learn more?
(Other definitions for purse that I've seen before include "Pocket" , "Small pouch for carrying money, mostly by women" , "Compress (lips)" , "Handbag or money prize" , "amount of money" .)