Moving down river current on ship (6)
I believe the answer is:
aboard
'on ship' is the definition.
(aboard a ship)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for aboard that I've seen before include "On (eg, a ship)" , "Joined for mutual benefit" , "On an aircraft" , "on base" , "A poet penning nothing on vessel" .)