A tepee leaky on the outside is serious (10)

I believe the answer is:

portentous

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'serious' is the definition.
(thesaurus)

'a tepee leaky on the outside' is the wordplay.
'a tepee' becomes 'tent' (I can't explain this - if you can you should believe this answer much more).
'leaky' becomes 'porous' ('porous' can be similar in meaning to 'leaky').
'on the outside' means one lot of letters goes inside another.
'tent' placed within 'porous' is 'PORTENTOUS'.

'is' is the link.

Can you help me to learn more?

(Other definitions for portentous that I've seen before include "Of great significance" , "Of ominous meaning" .)

I've seen this clue in The Globe and Mail.

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