Hengist's soldiers? (3)

I believe the answer is:

gis

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'soldiers?' is the definition.
(GI is an informal term for a US soldier)

'hengist's' is the wordplay.
'GIS' is hidden within the letters.
However, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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