Mathematical point of a currency system (7)

I believe the answer is:

decimal

I'm a little stuck... Click here to teach me more about this clue!

'mathematical point' is the definition.
'decimal' can be an answer for 'point' (I've seen this in another clue). I am not certain of the 'mathematical' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for decimal that I've seen before include "(System) based on tens" , "Sort of coinage" , "of Number Ten" , "Using powers of ten" , "fraction" .)

I've seen this clue in The Irish Independent.

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