Get upset, being four-fifths wrong! (4)

I believe the answer is:

grow

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'get' is the definition.
(synonyms)

I cannot understand how the remainder of the clue works.

(Other definitions for grow that I've seen before include "Multiply" , "Wax" , "Expand" , "Develop - propagate" , "Increase in size" .)

I've seen this clue in the Evening Standard.

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