Way you initially cross an underworld river (4)
I believe the answer is:
styx
'an underworld river' is the definition.
(I've seen this before)
'way you initially cross' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'initially' says to take the initial letters.
'cross' becomes 'x' (an X is a cross).
The initial letter of 'you' is 'y'.
'st'+'y'+'x'='STYX'
(Other definitions for styx that I've seen before include "River of hell" , "The last river one crosses" , "Underworld river with ferryman Charon" , "One of the rivers of Hades" , "Underworld river ferried by Charon" .)