Investment fund segment to rely on (4,5)
I believe the answer is:
unit trust
'investment fund' is the definition.
(I've seen this before)
'segment to rely on' is the wordplay.
'segment' becomes 'unit' (I've seen this in another clue).
'to rely on' becomes 'trust' ('trust' can be a synonym of 'rely on').
'unit'+'trust'='UNIT TRUST'
(Other definitions for unit trust that I've seen before include "Multiple investment scheme" , "Investment fund" , "Investment package" , "Form of investment" , "Type of investment" .)