It's a mistake to hesitate or become upset, right? (5)
I believe the answer is:
error
'it's a mistake' is the definition.
(synonyms)
'hesitate or become upset right?' is the wordplay.
'hesitate' becomes 'er' (expression used when hesitating).
'become' is an anagram indicator.
'upset right?' becomes 'r' (common abbreviation. I am not sure about the 'upset' bit.).
'or' with letters rearranged gives 'ro'.
'er'+'ro'+'r'='ERROR'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for error that I've seen before include "wrongdoing" , "Howler" , "wrong" , "Bug" , "1 down" .)