I've been stupid about beginning to save money (4)
I believe the answer is:
dosh
'money' is the definition.
(slang term)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for dosh that I've seen before include "currency" , "Ready" , "Money (sl.)" , "cash" , "Bread - gravy - dough" .)