Its beauty is but skin deep (6)

I believe the answer is:

veneer

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

'skin deep' is the definition.
'veneer' can be an answer for 'skin' (thesaurus). I am not sure about the 'deep' bit.

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

Can you help me to learn more?

(Other definitions for veneer that I've seen before include "Decorative coating" , "Thin skin on surface" , "Evener (anag.)" , "Superficial appearance" , "Thin wooden facing" .)

I've seen this clue in the Evening Standard.

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