Order digit finally to poke patient (5)

I believe the answer is:

caste

'order' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for caste that I've seen before include "Exclusive social class" , "Hereditary social class among Hindus" , "Hindu determinant of status" , "Level in society" , "Social class in Hinduism" .)

I've seen this clue in The Independent.

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