Reticence? Keep it for future use (7)

I believe the answer is:

reserve

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'reticence?' is the definition.
('reserve' can be a synonym of 'reticence')

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for reserve that I've seen before include "Husband - substitute" , "not in the front line" , "Book - substitute player" , "Modesty- store" , "ice" .)

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