Send back water, being somewhat green (5)
I believe the answer is:
naive
'somewhat green' is the definition.
'send back water' is the wordplay.
'send back' shows that the letters should be reversed in order.
'water' becomes 'evian' (mineral water brand).
'evian' backwards is 'NAIVE'.
'being' is the link.
(Other definitions for naive that I've seen before include "Inexperienced and guileless" , "Too simple and trusting" , "Showing lack of experience or sophistication" , "Showing lack of maturity or judgement" , "Too trusting and impractical" .)