This is a must for initial phases (5)

I believe the answer is:

alpha

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'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" .)

I've seen this clue in The Telegraph.

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