Chap’s tax-free savings account (5)

I believe the answer is:

isaac

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'chap's' is the definition.

'tax-free savings account' is the wordplay.
'tax-free savings' becomes 'isa' (I've seen this before. I am not sure about the 'tax-free' bit.).
'account' becomes 'ac' (abbreviation).
'isa'+'ac'='ISAAC'

Can you help me to learn more?

(Other definitions for isaac that I've seen before include "one almost sacrificed" , "- Newton, Eng. physicist and mathematician" , "Actor Oscar" , "-- Asimov, science fiction writer" , "Son almost sacrificed" .)

I've seen this clue in The Times.

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