Around town main road sure to be closed completely (8)

I believe the answer is:

airtight

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'closed completely' is the definition.
'airtight' can be an answer for 'closed'. I'm not sure about the 'completely' bit.

'around town main road sure' is the wordplay.
'around' indicates putting letters inside.
'town' becomes 't' (abbreviation for town).
'main road' becomes 'AI' (resembles Ar, major UK road).
'sure' becomes 'right' (I can't justify this - if you can you should give a lot more credence to this answer).
'ai'+'right'='airight'
't' put into 'airight' is 'AIRTIGHT'.

'to be' is the link.

Can you help me to learn more?

(Other definitions for airtight that I've seen before include "securely sealed" , "Firmly closed" , "Not allowing gas to pass" , "Impenetrable" , "Not allowing gas to enter or escape" .)

I've seen this clue in The Independent.

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