Crossword Genius

Joined forces in petrol refining (8)

Ross

I believe the answer is:

interpol

I'm a little stuck... Click here to teach me more about this clue!

'joined forces' is the definition.
(I've seen this before)

'petrol refining' is the wordplay.
I cannot quite see how this works, but
an anagram of 'petrol' is 'terpol' which is located in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'in' is the link.

Can you help me to learn more?

(Other definitions for interpol that I've seen before include "detectives" , "International police force" , "Lyon-based Police Commission" , "Organisation for international police cooperation" , "Cross-border crimefighters" .)

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