Make lighter colour blue (4,4)
I believe the answer is:
tone down
'make lighter' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'colour blue' is the wordplay.
'colour' becomes 'tone' (tone can mean a colour or shade).
'blue' becomes 'down' (both can mean sad).
'tone'+'down'='TONE DOWN'
Can you help me to learn more?
(Other definitions for tone down that I've seen before include "Mute" , "Make less harshly coloured" , "Temper" , "Moderate" , "D to C, perhaps?" .)