Not all invest in coin (4,5)
I believe the answer is:
half crown
'coin' is the definition.
(I know that half crown is a type of coin)
'not all invest' is the wordplay.
'not all' becomes 'half' (I can't justify this - if you can you should believe this answer much more).
'invest' becomes 'crown' (crowning is a kind of investing).
'half'+'crown'='HALF-CROWN'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for half crown that I've seen before include "bit out of date" , "Twelve and a half pence in old money" , "In the 17th century, Mary had this" , "Former silver coin" , "Former British coin" .)