Fool takes group inside property (6)
I believe the answer is:
assets
'property' is the definition.
(I've seen this before)
'fool takes group' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'takes' is an insertion indicator.
'group' becomes 'set' (both can mean a collection).
'ass' going around 'set' is 'ASSETS'.
'inside' is the link.
(Other definitions for assets that I've seen before include "One's total goods" , "Properties that have money value" , "Valuables, things worth having" , "Personal wealth" , "Useful or valuable possessions or qualities" .)