Become happier (5,2)
I believe the answer is:
cheer up
'become happier' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cheer up that I've seen before include "Become happier" , "Turn that frown upside down" , "Take encouragement from" , "Get happy, lose the blues" , "Don't be so glum!" .)