Water colour (4)
I believe the answer is:
aqua
This clue is a double definition.
'water' is the first definition.
(thesaurus)
'colour' is the second definition.
(aqua is a kind of colour)
(Other definitions for aqua that I've seen before include "Prefix denoting "water"" , "- Regia; - fortis" , "- - Libra" , "-- fortis, 'strong water'" , "Blue tinged with green" .)