Mathematician's ISA account recently opened with large amount (5,6)
I believe the answer is:
isaac newton
'mathematician's' is the definition.
(Isaac Newton is an example)
'isa account recently opened with large amount' is the wordplay.
'account' becomes 'ac' (abbreviation for account).
'recently opened' becomes 'new'.
'with' is a charade indicator (letters next to each other).
'large amount' becomes 'ton' (I've seen this in another clue).
'isa'+'ac'+'new'+'ton'='ISAAC NEWTON'
(Other definitions for isaac newton that I've seen before include "noted scientist" , "he described it mathematically" , "Seventeenth to eighteenth century English physicist/mathematician" , "Principia author" , "Laws of Motion man" .)