Never-ending consumption of Twitter is superficial (8)

I believe the answer is:

external

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

'is superficial' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for external that I've seen before include "involving foreign nations" , "abroad" , "Outward" , "Relating to the outside" , "Lying outside" .)

I've seen this clue in the Sydney Morning Herald.

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