Crossword Genius

Source of oil for an articulated truck (3)

Ross

I believe the answer is:

rig

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'an articulated truck' is the definition.
(I know that semi-trailer truck can be written as rig)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for rig that I've seen before include "Fit with sails and tackling" , "Outfit of clothes or oil-drilling platform" , "Accoutre" , "Manipulate fraudulently" , "Masts, spars and sails on a vessel" .)

I've seen this clue in the Evening Standard.
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