Understood as text messages are? (8)

I believe the answer is:

unspoken

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

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

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

(Other definitions for unspoken that I've seen before include "Tacit" , "as text messaging is, for example" , "Not made explicit" , "Not expressed" , "Understood" .)

I've seen this clue in The Sun.

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