A big river, mostly deep blue (4)
I believe the answer is:
anil
'deep blue' is the definition.
(blue dye and its specific colour)
'a big river mostly' is the wordplay.
'big river' becomes 'Nile' (River Nile).
'mostly' means to remove the last letter (most but not all of the word).
'nile' with its last letter removed is 'nil'.
'a'+'nil'='ANIL'
(Other definitions for anil that I've seen before include "Shrub providing indigo" , "Indigo (plant or dye)" , "West Indian shrub, source of indigo" , "Shrub yielding indigo" , "Colouring" .)