Sealed off main road just outside entrance to town (8)
I believe the answer is:
airtight
'sealed off' is the definition.
(airtight means sealed so that air cannot enter)
'main road just outside entrance to town' is the wordplay.
'main road' becomes 'AI' (resembles A1, major UK road).
'just' becomes 'right' (associated in meaning).
'outside' means one lot of letters goes inside another.
'entrance to' says to take the initial letters.
The initial letter of 'town' is 't'.
'right' enclosing 't' is 'rtight'.
'ai'+'rtight'='AIRTIGHT'
(Other definitions for airtight that I've seen before include "Firmly closed" , "securely sealed" , "Without weak points" , "Sure" , "Not allowing gas to pass" .)