Ferryman takes companion forward, crossing a river (6)
I believe the answer is:
charon
'ferryman' is the definition.
(ferryman of Hades in Greek myth)
'companion forward crossing a river' is the wordplay.
'companion' becomes 'ch' (I've seen this in other clues).
'forward' becomes 'on'.
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'ch'+'on'='chon'
'a'+'r'='ar'
'chon' placed around 'ar' is 'CHARON'.
'takes' acts as a link.
(Other definitions for charon that I've seen before include "underground transporter" , "Boatman across the river Styx" , "He ferries dead over Styx" , "Mythical boatman of the River Styx" , "Ferryman on river of Hades" .)