Spend a lot, throwing pound into stream (6,3)
I believe the answer is:
splash out
'spend a lot throwing' is the definition.
I can't tell whether this defines the answer.
'pound into stream' is the wordplay.
'pound' becomes 'lash' (I can't justify this - if you can you should believe this answer much more).
'into' means one lot of letters goes inside another.
'stream' becomes 'spout' (spout can mean to stream or spurt).
'lash' placed into 'spout' is 'SPLASH OUT'.
Can you help me to learn more?
(Other definitions for splash out that I've seen before include "Blow brass" , "Spend money freely" , "Spend a lot of money" , "extravagant" .)