Das Boot typed with thumbs? It's not obvious to the reader (7)

I believe the answer is:

subtext

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

'das' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for subtext that I've seen before include "Implicit meaning" , "Something implied" , "Unwritten meaning" , "Underlying theme" , "Level of deeper meaning" .)

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