Downhearted jester! (5)
I believe the answer is:
clown
'jester' is the definition.
(I know that clown is a type of jester)
'downhearted' is the wordplay.
I cannot really see how this works, but
'downhearted' could be 'low' (synonyms) and 'low' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for clown that I've seen before include "Comic entertainer; fool" , "Act comically" , "Circus performer" , "Jester" , "Buffoon" .)