Man’s tax-free account about to rise (5)
I believe the answer is:
isaac
'man's' is the definition.
(I've seen this before)
'tax-free account about to rise' is the wordplay.
'tax-free account' becomes 'isa' (Individual Savings Account).
'about' becomes 'ca' (circa).
'to rise' says the letters should be written backwards.
'ca' back-to-front is 'ac'.
'isa'+'ac'='ISAAC'
(Other definitions for isaac that I've seen before include "Biblical father of twins" , "one almost sacrificed" , "Esau's father" , "Sacrificial offering" , "Father to Jacob and Esau" .)