Structure in support of motorway (4)
I believe the answer is:
form
'structure' is the definition.
(form is a kind of structure)
'in support of motorway' is the wordplay.
'in support of' becomes 'for' (as in being for a particular cause).
'motorway' becomes 'M' (as in motorway names - Mf, M4 etc).
'for'+'m'='FORM'
(Other definitions for form that I've seen before include "Printed document with blank spaces for information to be inserted" , "Class in a school, or the shape of something" , "Class; schedule" , "A hare's hide-out" , "Fitness" .)