Law: handrails must be fitted for this ancient structure (8,4)
I believe the answer is:
hadrians wall
'this ancient structure' is the definition.
(wall built by the Roman emperor Hadrian)
'law handrails must be fitted' is the wordplay.
'must be fitted' indicates anagramming the letters (letters refitted or rearranged).
'law'+'handrails'='lawhandrails'
'lawhandrails' anagrammed gives 'HADRIANS WALL'.
'for' acts as a link.
(Other definitions for hadrians wall that I've seen before include "Roman fortification across Britain" , "Old border" , "Roman fortification in northern England" , "Solid defence" , "Roman defence against Scots and Picts" .)