Depth marker is near middle of hot tub (7)

I believe the answer is:

isobath

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'depth marker' is the definition.
The definition suggests a singular noun which matches the answer.

'is near middle of hot tub' is the wordplay.
'near' says to put letters next to each other.
'middle of' indicates the central letters.
'tub' becomes 'bath' (synonyms).
The middle letter of 'hot' is 'o'.
'is'+'o'+'bath'='ISOBATH'

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