Eternal promise of face-cream salesmen? (7)

I believe the answer is:

ageless

I'm a little stuck... Click here to teach me more about this clue!

'eternal' is the definition.
(I know that eternal can be written as ageless)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ageless that I've seen before include "Staying young" , "not appearing to get old" , "Eternally young" , "not getting on" , "impossible to date" .)

I've seen this clue in The Sun.

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