Knocking back green water? (5)
I believe the answer is:
evian
'water?' is the definition.
(mineral water brand)
'knocking back green' is the wordplay.
'knocking back' says the letters should be written backwards.
'green' becomes 'naive' (green can mean naive or innocent).
'naive' written backwards gives 'EVIAN'.
(Other definitions for evian that I've seen before include "springwater" , "This water" , "spa town" .)