Mercy! Not half! (7)

I believe the answer is:

quarter

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This might be a double definition.

'mercy' is the first definition.
(I know that quarter is a type of mercy)

'not half' is the second definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps there's an association between them I don't understand?

Can you help me to learn more?

(Other definitions for quarter that I've seen before include "Fourth; part of town" , "A fourth - city district" , "Clemency" , "House" , "US coin" .)

I've seen this clue in The Irish News.

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