One has confidence in this investment (4,5)
I believe the answer is:
unit trust
'this investment' is the definition.
'one has confidence' is the wordplay.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'has' is a charade indicator (letters next to each other).
'confidence' becomes 'trust' ('trust' can be a synonym of 'confidence').
'unit'+'trust'='UNIT TRUST'
'in' is the link.
(Other definitions for unit trust that I've seen before include "Form of investment" , "Investment system" , "Multiple investment scheme" , "Investment package" , "Investment fund" .)