Come out of the blue? (5,2)
I believe the answer is:
cheer up
'come out of the blue?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cheer up that I've seen before include "Become happier" , "Take encouragement from" , "Get happy, lose the blues" , "Cause (someone) to feel happier" , "Don't be so miserable!" .)