Green where spring water comes from, seen travelling west (5)
I believe the answer is:
naive
'green' is the definition.
(similar in meaning)
'where spring water comes from seen travelling west' is the wordplay.
'where spring water comes from' becomes 'Evian' (Evian-les-Bains in France is a source of water).
'seen travelling west' is a reversal indicator (letters written right to left or 'east to west').
'evian' reversed gives 'NAIVE'.
(Other definitions for naive that I've seen before include "Short on experience" , "Natural, unaffected" , "Jejune" , "Unsophisticated or inexperienced" , "Rather too simple and gullible" .)