Dominant part in final phase (5)
I believe the answer is:
alpha
'dominant' is the definition.
(I've seen this before)
'part in final phase' is the wordplay.
'part in' indicates a hidden word.
'ALPHA' is hidden within 'final phase'.
(Other definitions for alpha that I've seen before include "A for Aristophanes!" , "very good rating" , "Most prominent star" , "Foreign character" , "leader in Athens, originally" .)