Water? A quart? Not right! (4)
I believe the answer is:
aqua
'water?' is the definition.
(I know that water can be written as aqua)
'a quart? not right' is the wordplay.
'not' indicates named letters should be taken away.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'quart' with 'rt' taken out is 'qua'.
'a'+'qua'='AQUA'
(Other definitions for aqua that I've seen before include "Water (L)" , "Water; pale blue colour" , "(Denoting) water (Lat)" , "Latin for water" , "which supports boats?" .)