Nancy's mother in shallow lake (4)

I believe the answer is:

mere

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'lake' is the definition.
(I know that mere is a type of lake)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mere that I've seen before include "Bare, simple" , "Nothing more than is specified - or a poetic lake" , "Just an English lake" , "Paltry" , "Pure; lake" .)

I've seen this clue in The Telegraph and The Sun.

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