Group about to set off (5)

I believe the answer is:

caste

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'group' is the definition.
(an ethnic or social group)

I can't explain the remainder of the clue.

(Other definitions for caste that I've seen before include "A sect (anag)" , "place in society" , "Social order" , "Religious or social class" , "Social division in Hinduism" .)

I've seen this clue in The Mirror.

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