A river that's partly blue, almost indigo (4)
I believe the answer is:
anil
'indigo' is the definition.
(anil is another word for indigo)
'a river that's partly blue almost' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for anil that I've seen before include "It produces dye" , "Colouring" , "Indian man" , "Shrub yielding indigo" , "West Indian shrub, source of indigo" .)