Crossword Genius

The sentry lost his head going in (5)

Ross

I believe the answer is:

entry

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

'going in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'the sentry lost his head' is the wordplay.
'lost his head' means to remove the first letter.
'sentry' with its initial letter removed is 'ENTRY'.

Can you help me to learn more?

(Other definitions for entry that I've seen before include "List of competitors" , "Competitor" , "Opening" , "Door; appearance" , "List of candidates" .)

I've seen this clue in The Globe and Mail.
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