Officer casing area dismissed for taking unrefined coke? (4-5)

I believe the answer is:

coal-fired

'for taking unrefined coke?' is the definition.
(powered by coal - coke is made by heating coal)

'officer casing area dismissed' is the wordplay.
'officer' becomes 'col' (short for colonel).
'casing' indicates putting letters inside (I've seen this in other clues).
'area' becomes 'a' (maths abbreviation).
'dismissed' becomes 'fired' (synonyms).
'col' enclosing 'a' is 'coal'.
'coal'+'fired'='COAL-FIRED'

I've seen this clue in The Guardian.

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