What Wogan jocularly recalled as "river from hell" (5)

I believe the answer is:

lethe

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'river from hell' is the definition.
'lethe' can be an answer for 'river' (I've seen this before). I am not certain of the 'from hell' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for lethe that I've seen before include "where elephants never drink?" , "River whose waters caused forgetfulness" , "Ethel's mythical river of forgetfulness" , "Oblivion; river of the underworld" , "What could make one forget" .)

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