Set sail on Eastern passage (5)
I believe the answer is:
aisle
'passage' is the definition.
(aisle can mean a path or passageway)
'set sail on eastern' is the wordplay.
'set' indicates an anagram (instruction to set the letters).
'on' says to put letters next to each other.
'eastern' becomes 'e'.
'sail' anagrammed gives 'aisl'.
'aisl'+'e'='AISLE'
(Other definitions for aisle that I've seen before include "Part of supermarket" , "Extension of the nave in a church" , "The way to get married in church?" , "Walkway between seats" , "way to the altar?" .)