Permit man to see underworld river (5)
I believe the answer is:
lethe
'underworld river' is the definition.
(I've seen this before)
'permit man' is the wordplay.
'permit' becomes 'let' (synonyms).
'man' becomes 'he' (he refers to a man).
'let'+'he'='LETHE'
'to see' is the link.
(Other definitions for lethe that I've seen before include "Mythological river in Hades" , "where elephants never drink?" , "River whose waters caused forgetfulness" , "What could make one forget" , "In Greek myth, the river of forgetfulness flowing through Hades" .)