Any upset can be discouraging (3)
I believe the answer is:
nay
'be discouraging' is the definition.
I can't judge whether this definition defines the answer.
'any upset' is the wordplay.
'upset' indicates anagramming the letters.
'any' is an anagram of 'NAY'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for nay that I've seen before include "telling a nicker from a pony?" , "Negative answer" , "In point of fact - a parliamentary vote against" , "Denial" , "No (dialect); or rather" .)