Exercise in order to be cheerful? (4-2)
I believe the answer is:
chin-up
I believe this clue is a double definition.
'exercise' is the first definition.
(I know that chin-up is a type of arm exercise)
'order to be cheerful?' is the second definition.
('chin up' means 'cheer up')
'in' is the link.
(Other definitions for chin-up that I've seen before include "Remain cheerful, elevate mandible" , "Keep cheerful! (colloq.)" , "Show courage with elevated mandible" , "Head posture to be brave, remain cheerful" , "Exhortation to stay cheerful in difficult circumstances" .)