Down below, so it could be better (5)

I believe the answer is:

under

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'down below' is the definition.
(synonyms)

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

(Other definitions for under that I've seen before include "Covered" , "Subordinate to; planted with" , "Anaesthetised" , "Less than" , "Governed by" .)

I've seen this clue in the Evening Standard.

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