Thus, during January, he partnered Medea (5)
I believe the answer is:
jason
'he partnered medea' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'thus during january' is the wordplay.
'thus' becomes 'so' (synonyms).
'during' indicates putting letters inside.
'january' becomes 'Jan' (synonyms).
'so' going inside 'jan' is 'JASON'.
Can you help me to learn more?
(Other definitions for jason that I've seen before include "Leader of the Argonauts who sailed in quest of the Golden Fleece" , "He sailed the Argo to find the Golden Fleece" , "He sought the Golden Fleece" , "He led the Argonauts to find the Golden Fleece" , "Boy" .)