I say, get a move on (3)

I believe the answer is:

gee

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'i' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gee that I've seen before include "Expression of mild surprise (mainly US)" , "Alphabet's seventh letter" , "Mild expression of surprise, may go before whiz" , "Gracious" , "Golly" .)

I've seen this clue in The Independent.

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