Take heart young man. Out of bed! (4,2)
I believe the answer is:
buck up
'take heart' is the definition.
(synonyms)
'young man out of bed' is the wordplay.
'young man' becomes 'buck' (old-fashioned term for a young man).
'out of bed' becomes 'up' (synonyms).
'buck'+'up'='BUCK UP'
(Other definitions for buck up that I've seen before include "Hurry or become more cheerful - like wild horse?" , "Become more serious" , "Take heart" , "Become more cheerful like unruly horse" , "Informally become more cheerful or make haste - like restless horse?" .)