Constellation of water (around one litre) (6)
I believe the answer is:
aquila
'constellation' is the definition.
(I know that Aquila is a constellation)
'water around one litre' is the wordplay.
'water' becomes 'aqua' (I've seen this before).
'around' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'litre' becomes 'L' (abbreviation).
'i'+'l'='il'
'aqua' placed around 'il' is 'AQUILA'.
'of' is the link.
(Other definitions for aquila that I've seen before include "Italian city" , "Stars gathered together" , "Altair's constellation" , "group of stars" , "starry set" .)