Eternal theme of the beautician (7)

I believe the answer is:

ageless

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

I can't explain the rest of the clue.

(Other definitions for ageless that I've seen before include "Lasting a long time" , "Sea-legs (anag.)" , "not appearing to get old" , "Not appearing to grow older" , "impossible to date" .)

I've seen this clue in The Guardian and The Globe and Mail.

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