Real phantom conceals character (5)
I believe the answer is:
alpha
'character' is the definition.
(I know that Alpha is a Greek letter)
'real phantom conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'ALPHA' is hidden in the letters of 'real phantom'.
(Other definitions for alpha that I've seen before include "leader in Athens, originally" , "A for Aristophanes!" , "very good rating" , "First in order of importance (for a Greek?)" , "best score" .)