Fool holds fixed investments? (6)
I believe the answer is:
assets
'investments?' is the definition.
(investment is a kind of asset)
'fool holds fixed' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'holds' indicates putting letters inside.
'fixed' becomes 'set' (both can mean decided or determined).
'ass' placed around 'set' is 'ASSETS'.
(Other definitions for assets that I've seen before include "(Person's) property" , "One's valuable possessions" , "Total value of possessions" , "Personal wealth" , "Useful or valuable things" .)