In principle, there's oblivion in it (5)
I believe the answer is:
lethe
'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?" .)