Highly acidic character (3)

I believe the answer is:

phi

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'character' is the definition.
(I know that Greek letter is a type of character)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for phi that I've seen before include "Greek letter in previous answer" , "Greek letter; sounds like expression of outrage" , "Greek letter; hip (anag.)" , "exotic character" , "Letter from the Balkans" .)

I've seen this clue in The Times.

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