Water thrown back, so wet behind the ears (5)
I believe the answer is:
naive
'wet behind the ears' is the definition.
(I've seen this in another clue)
'water thrown back' is the wordplay.
'water' becomes 'evian' (mineral water brand).
'thrown back' says the letters should be written in reverse.
'evian' back-to-front is 'NAIVE'.
'so' acts as a link.
(Other definitions for naive that I've seen before include "Callow" , "Jejune" , "Inexperienced, innocent" , "Unenlightened" , "Inexperienced or gullible" .)