Difficult to get cheerful when poor (4,2)
I believe the answer is:
hard up
'poor' is the definition.
(similar in meaning)
'difficult to get cheerful' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'to get' is a charade indicator (letters next to each other).
'cheerful' becomes 'up' (up can mean cheerful or upbeat).
'hard'+'up'='HARD-UP'
'when' acts as a link.
(Other definitions for hard up that I've seen before include "Short of money - it's tough at the top" , "Having little money, almost skint" , "Impoverished" , "boracic" , "Poor" .)