Mark is accompanying Elizabeth, pioneer (9)

I believe the answer is:

coloniser

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'pioneer' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'mark is accompanying elizabeth' is the wordplay.
'mark' becomes 'colon' (colon is a kind of punctuation mark).
'accompanying' is a charade indicator (letters next to each other).
'elizabeth' becomes 'ER'.
'colon'+'is'+'er'='COLONISER'

Can you help me to learn more?

(Other definitions for coloniser that I've seen before include "One's part of the settled establishment" , "Settler in new area" .)

I've seen this clue in The Times.

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