River in which Achilles was dipped (4)
I believe the answer is:
styx
'river in which achilles was dipped' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for styx that I've seen before include "Charon's 26 carried dead souls across this river" , "One of the underworld rivers" , "River around Hades" , "The last river one crosses" , "River crossed at death" .)