Invest old money (5)
I believe the answer is:
crown
This might be a double definition.
'invest old' is the first definition.
'crown' can be an answer for 'invest' (crowning is a kind of investing). I'm not certain of the 'old' bit.
'money' is the second definition.
(I know that crown is a type of coin)
Can you help me to learn more?
(Other definitions for crown that I've seen before include "'Uneasy lies the head that wears a . . . . .'" , "Symbol of royalty" , "Royal headdress" , "Old coin; top of tree" , "peak" .)