Injure husband and wife on westbound major route (6)
I believe the answer is:
impair
'injure' is the definition.
(I've seen this in another clue)
'husband and wife on westbound major route' is the wordplay.
'husband and wife' becomes 'pair' (I've seen this before).
'on' says to put letters next to each other.
'westbound' says the letters should be written backwards (letters written from right to left).
'major route' becomes 'mi' (resembles M1, English motorway).
'mi' written backwards gives 'im'.
'pair' put after 'im' is 'IMPAIR'.
(Other definitions for impair that I've seen before include "Spoil strength, quality of" , "Hobble" , "Harm" , "Spoil, degrade" , "Hinder" .)