Mythical one restrained in speech (4)
I believe the answer is:
'mythical one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'restrained in speech' is the wordplay.
'restrained' becomes 'low-key' (similar in meaning).
'in speech' indicates a 'sounds like' (homophone) clue.
'lowkey' sounds like 'LOKI'.
Can you help me to learn more?
(Other definitions for loki that I've seen before include "Norse god of mischief", "Gossip".)