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This is a must for initial phases (5)

Ross

I believe the answer is:

alpha

I'm a little stuck... Click here to teach me more about this clue!

'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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