Returning by way of Northeast Passage is simple (5)
I believe the answer is:
naive
'is simple' is the definition.
(similar in meaning)
'returning by way of northeast' is the wordplay.
'returning' says the letters should be written backwards.
'by' becomes 'via' (travelling by or via a place).
'way' becomes 'n' (Compass direction - North).
'of northeast' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'via'+'n'='vian'
'vian' reversed gives 'naiv'.
'naiv'+'e'='NAIVE'
'passage' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for naive that I've seen before include "Ingenuous and gullible" , "Simple, innocent or unsophisticated" , "Unsophisticated and gullible" , "Wet behind ears" , "Rather too simple and gullible" .)