Crossword Genius

Fantastic gene tea offers youth (7)

Ross

I believe the answer is:

teenage

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'fantastic' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for teenage that I've seen before include "Eat gene to become young" , "Like a category of youngsters" , "Suitable for years 13-19" , "Juvenile" , "of seven difficult years" .)

I've seen this clue in The Sun.
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