Tar that's mine, but needing a stir (7)

I believe the answer is:

bitumen

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

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

'mine but needing a stir' is the wordplay.
'mine but' can be anagrammed to 'BITUMEN'.
Though, I am unsure how this is indicated.

'that's' is the link.

Can you help me to learn more?

(Other definitions for bitumen that I've seen before include "Mixed-up tar" , "material that surfaces" , "Material for road surfacing, roofing, etc" , "Road-surfacing material" , "might be used to 16 [RESURFACE]" .)

I've seen this clue in The Sun.

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