Journey is a dopey thing to do (4)
I believe the answer is:
This clue is a double definition.
'journey' is the first definition.
(I know that journey is a more specific form of the action trip)
'a dopey thing to do' is the second definition.
(as in tripping on drugs - dope is a drug)
'is' acts as a link.
(Other definitions for trip that I've seen before include "a result of drug-taking" , "Hallucinatory experience" , "Catch one's foot" , "often occurring just before the fall" , "Short excursion" .)