Which person? (4)
'which person?' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for who that I've seen before include "Which people" , "international medical group" , "Question of identity" , "The - - , rock legends" , "medical experts" .)
(Other definitions for whom that I've seen before include "One seeking to influence MPs" , "Objectively who" .)