The unclued lights are all CHARACTERS (18D) in Plato’s dialogues, all but SOCRATES (1A) appearing in titles. In six cells, clashing letters could be combined to form letters of the Greek alphabet (e.g. LAMB + DA = LAMBDA) — these six characters spell out the name PLATO in Greek (Πλάτων, using lower case letters, was also acceptable).
 
First prize Sandra Speak, Dursley, Glos
Runners-up S.J.J. Tiffin, Cockermouth, Cumbria; M. Puttick, Montpezat, France