Later, in the court case, becomes ternary (10)
I believe the answer is:
trilateral
'ternary' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'later in the court case' is the wordplay.
'in the' indicates putting letters inside.
'court case' becomes 'trial' (I've seen this in another clue).
'later' inserted inside 'trial' is 'TRILATERAL'.
'becomes' acts as a link.
Can you help me to learn more?
(Other definitions for trilateral that I've seen before include "Three-sided" , "Figure" , "involving several parties" .)