Blue Arizona river (5)
I believe the answer is:
azure
'blue' is the definition.
(azure is a kind of blue)
'arizona river' is the wordplay.
'arizona' becomes 'az' (abbreviation).
'river' becomes 'ure' (river in Yorkshire).
'az'+'ure'='AZURE'
(Other definitions for azure that I've seen before include "Heraldic blue" , "Sky-coloured" , "Cerulean" , "A bright sky-blue" , "Description of sky, perhaps" .)