Leading Greek character in final phase (5)
I believe the answer is:
alpha
'leading greek character' is the definition.
(first letter of the Greek alphabet)
'in final phase' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALPHA' is hidden in the letters of 'final phase'.
(Other definitions for alpha that I've seen before include "Brightest star" , "star outshining the others" , "Beginning" , "First-rate" , "Greek vowel" .)