What does the cryptic crossword clue "One joining Plato in discursive medium (3,5)" mean?

The definition is "medium". The wordplay uses an anagram. anagram of I PLATO IN = OILPAINT The answer is OILPAINT.

One joining Plato in discursive medium (3,5)

Guardian Cryptic #29568 | 14 Across | 2024-12-17 flower