Posted some US currency in grounds (8)
I believe the answer is:
sediment
'grounds' is the definition.
(thesaurus)
'posted some us currency in' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'some us currency' becomes 'dime' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'sent' enclosing 'dime' is 'SEDIMENT'.
Can you help me to learn more?
(Other definitions for sediment that I've seen before include "Grounds - debris" , "Residue" , "Dregs; deposit" , "I'm nested in mud" , "Matter settling out in liquid" .)