Electronic ticket is sound protocol (9)

I believe the answer is:

etiquette

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

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

I cannot understand how the remainder of the clue works.

(Other definitions for etiquette that I've seen before include "Behavioural code" , "Rules of correct social behaviour" , "Conventional courtesy" , "Protocol" , "Decorum, ceremony" .)

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

Want a hint initially instead of a full solution? Install my app