At heart, clown is quite sad (3)
I believe the answer is:
low
'quite sad' is the definition.
'low' can be an answer for 'sad' (I've seen this in other clues). I'm not sure about the 'quite' bit.
'at heart clown is' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'LOW' is hidden in the letters of 'clown is'.
Can you help me to learn more?
(Other definitions for low that I've seen before include "Deep sound of cattle - not high" , "SECOND PART OF 14" , "Dejected; moo" , "Humble; dejected" , "Deep; moo" .)