Grand gold box is most ungainly (8)

I believe the answer is:

gauchest

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'most ungainly' is the definition.
Both the definition and answer are superlative adjectives. Maybe there's a link between them I don't understand?

'grand gold box' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'box' becomes 'chest' (chest is a kind of box).
'g'+'au'+'chest'='GAUCHEST'

'is' acts as a link.

Can you help me to learn more?

I've seen this clue in The Times.

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