Rust a blade knocked off railing (10)
I believe the answer is:
balustrade
'railing' is the definition.
(balustrade is a kind of railing)
'rust a blade knocked off' is the wordplay.
'knocked off' indicates an anagram.
'rust'+'a'+'blade'='rustablade'
'rustablade' with letters rearranged gives 'BALUSTRADE'.
(Other definitions for balustrade that I've seen before include "Row of short pillars with coping" , "Railing on stairs or balcony" , "Row of pillars under rail" , "Handrail" , "Railing at the side of a balcony" .)