Green water, ebbing (5)
I believe the answer is:
naive
'green' is the definition.
(green can mean naive or innocent)
'water ebbing' is the wordplay.
'water' becomes 'evian' (mineral water brand).
'ebbing' says the letters should be written backwards (to ebb can mean to go backwards).
'evian' backwards is 'NAIVE'.
(Other definitions for naive that I've seen before include "Credulous, unworldly" , "Simple and guileless" , "Lacking wisdom and judgement" , "Foolish, inexperienced" , "Unaffected; credulous" .)