Biblical patriarch's investment account (5)
I believe the answer is:
isaac
'biblical patriarch's' is the definition.
(I've seen this before)
'investment account' is the wordplay.
'investment' becomes 'isa' (Individual Savings Account).
'account' becomes 'ac' (abbreviation for account).
'isa'+'ac'='ISAAC'
(Other definitions for isaac that I've seen before include "Singer" , "Biblical father of twins" , "Father to Jacob and Esau" , "Son prepared for sacrifice" , "designated victim" .)