Natural water flowing north (5)
I believe the answer is:
naive
'natural' is the definition.
(thesaurus)
'water flowing north' is the wordplay.
'water' becomes 'evian' (mineral water brand).
'flowing north' is a reversal indicator (in a down clue, letters go up).
'evian' back-to-front is 'NAIVE'.
(Other definitions for naive that I've seen before include "Too credulous or trusting" , "Too trusting and impractical" , "childlike" , "Jejune" , "Unsophisticated or inexperienced" .)