Achilles was dipped in it (4)
I believe the answer is:
styx
'achilles was dipped in it' 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 "Mythological river ferried by Charon" , "Mythological river in Hades" , "Infernal runner" , "Hell's river" , "River surrounding Hades" .)