’80s child has shed pounds for a very long time (9)

I believe the answer is:

millennia

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'a very long time' is the definition.

I can't explain the remainder of the clue.

(Other definitions for millennia that I've seen before include "Thousand-year periods" , "Could seem an interminable time" , "2000 years minimum" , "protracted periods" , "Periods of 1000 years" .)

I've seen this clue in The Guardian.

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