"Essential to contain it" I alleged at the beginning (7)

I believe the answer is:

initial

'at the beginning' is the definition.
(I've seen this before)

'essential to contain it i alleged' is the wordplay.
'essential to' indicates the answer is hidden within the clue (I've seen this in other clues).
'INITIAL' is hidden within 'contain it i alleged'.

(Other definitions for initial that I've seen before include "Name's first letter" , "Beginning - first letter" , "Start of name" , "Introductory" , "Approve" .)

I've seen this clue in The Independent.

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