Comedian is let down (5-2)
I believe the answer is:
stand-up
'comedian' is the definition.
(I've seen this before)
'let down' is the wordplay.
'let' becomes 'stand' (standing is a kind of letting).
'down' becomes 'up' (I can't justify this - if you can you should give a lot more credence to this answer).
'stand'+'up'='STAND-UP'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for stand-up that I've seen before include "Performed solo (of a comedy routine)" , "Form of comedy" , "Type of comedian" , "Sort of comic (to be counted?)" .)