Ship's officer therefore rings RAC up, following matter arising (10)

I believe the answer is:

supercargo

'ship's officer' is the definition.
(supercargo is a kind of ship's officer)

I don't understand how the rest of the clue works.

(Other definitions for supercargo that I've seen before include "Merchant navy officer responsible for what a ship is carrying" , "Ship's officer" , "Owner's representative on merchant ship" .)

I've seen this clue in The Guardian.

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