Simple — water's receding (5)
I believe the answer is:
naive
'simple' is the definition.
(similar in meaning)
'water's receding' is the wordplay.
'water' becomes 'evian' (mineral water brand).
'receding' says the letters should be written backwards.
'evian' written backwards gives 'NAIVE'.
(Other definitions for naive that I've seen before include "Too trusting and impractical" , "Rather too simple and gullible" , "Simple, innocent or unsophisticated" , "Lacking wisdom and judgement" , "Simple in style or behaviour" .)