Vacuous eejit in ferry might feel sick when they go to sea (3,4)
I believe the answer is:
set sail
'go to sea' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for set sail that I've seen before include "Embark on a voyage by ship" , "Leave port (by ship)" , "left land behind" , "have started journey" , "Left port" .)