Form of investment, one thing to have faith in (4,5)
I believe the answer is:
unit trust
'form of investment' is the definition.
(I've seen this before)
'one thing to have faith in' is the wordplay.
'one thing' becomes 'unit' (I have seen 'Single thing' mean 'unit' so perhaps 'thing' could also mean 'unit').
'to have faith in' becomes 'trust' ('trust' can be a synonym of 'have faith in').
'unit'+'trust'='UNIT TRUST'
(Other definitions for unit trust that I've seen before include "Investment system" , "Form of investment" , "Multiple investment scheme" , "Type of investment" , "Investment fund" .)