Clown at heart is depressed (3)
I believe the answer is:
low
'is depressed' is the definition.
('low' can be similar in meaning to 'depressed')
'clown at heart' is the wordplay.
'at heart' means to look at the middle letters.
The central letters of 'clown' are 'LOW'.
(Other definitions for low that I've seen before include "Dejected; moo" , "Without great height" , "Depressed; near the ground" , "Unimportant sort of owl" , "Inferior; moo" .)