A Whitehall success? (3)

I believe the answer is:

hit

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

'success?' is the definition.
(a hit is a commercial success)

'a whitehall' is the wordplay.
'HIT' is hidden within the letters.
But, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for hit that I've seen before include "turkey? No" , "dose of illegal drug" , "Knock" , "winner" , "recording in the charts" .)

I've seen this clue in the Evening Standard.

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