Colour is variable in a river (5)
I believe the answer is:
azure
'colour' is the definition.
(azure is a shade of blue)
'variable in a river' is the wordplay.
'variable' becomes 'z' (algebra).
'in' is an insertion indicator.
'river' becomes 'ure' (river in Yorkshire).
'a'+'ure'='aure'
'z' put within 'aure' is 'AZURE'.
'is' is the link.
(Other definitions for azure that I've seen before include "Deep blue colour" , "Description of sky, perhaps" , "Cerulean" , "Light blue" , "Delicate blue" .)