River not shallow but endless (3)

I believe the answer is:

dee

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'river' is the definition.
(River Dee)

'not shallow but endless' is the wordplay.
'not shallow' becomes 'de' (I can't justify this - if you can you should believe this answer much more).
'but' becomes 'yet' (synonyms).
'endless' indicates the central letters.
The middle of 'yet' is 'e'.
'de'+'e'='DEE'

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(Other definitions for dee that I've seen before include "River at Chester" , "Chester's river" , "Aberdeenshire river" , "John ---, Elizabethan mage" , "Scottish river" .)

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