US sect quiet beside a motorway (5)
I believe the answer is:
amish
'us sect' is the definition.
(the Amish are a religious community of the US)
'quiet beside a motorway' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'beside' says to put letters next to each other (I've seen this in other clues).
'motorway' becomes 'MI' (resembles M1, English motorway).
'a'+'mi'='ami'
'sh' put after 'ami' is 'AMISH'.
(Other definitions for amish that I've seen before include "Religious group" , "sectarian" , "Relating to a strict American sect" , "strict Pennsylvanian?" , "those eshewing modern technology" .)