One is or used to be (4)

I believe the answer is:

were

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'used to be' is the definition.
(eg 'you were...' means 'you used to be...')

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for were that I've seen before include "SECOND PART OF 9" , "'Are, in the past (4)'" , "(You/we/they) existed" , "(We/you) existed" , "Used to be" .)

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