Superior's bottom is a huge one (4,3)

I believe the answer is:

lake bed

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'a huge one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'superior's bottom' is the wordplay.
'superior' becomes 'lake' (I've seen this in another clue).
'bottom' becomes 'bed' (synonyms).
'lake'+'bed'='LAKE BED'

'is' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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