This is a must for initial phases (5)
I believe the answer is:
alpha
'this is a' is the definition.
(the Greek letter A)
'must for initial phases' is the wordplay.
'ALPHA' is hidden within the letters.
But, I am uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for alpha that I've seen before include "Brightest star" , "piece of Greek writing" , "Top star" , "high mark" , "European character" .)