Journey rubbish mostly (4)
I believe the answer is:
trip
'journey' is the definition.
(I know that journey can be written as trip)
'rubbish mostly' is the wordplay.
'rubbish' becomes 'tripe' (both can mean nonsense).
'mostly' means to remove the last letter.
'tripe' with its final letter removed is 'TRIP'.
(Other definitions for trip that I've seen before include "blunder" , "Excursion (while on drugs?)" , "Jaunt" , "Stumble - excursion" , "Journey; make a mistake" .)