A hot-tempered expression (4)
I believe the answer is:
oath
'expression' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?
'a hot-tempered' is the wordplay.
'tempered' indicates an anagram (temper can mean to adjust or modify).
'a'+'hot'='ahot'
'ahot' with letters rearranged gives 'OATH'.
Can you help me to learn more?
(Other definitions for oath that I've seen before include "Curse or solemn word" , "Curse - sworn statement" , "Tedious task" , "It's taken in court" , "A curse or a vow" .)