Crossword Genius

In principle, there's oblivion in it (5)

Ross

I believe the answer is:

lethe

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

'in principle there's oblivion' is the wordplay.
'in' indicates a hidden word.
'LETHE' is hidden within 'principle theres oblivion'.

'in' acts as a link.

Can you help me to learn more?

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

I've seen this clue in The Irish News.
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