Adults are moved behind black railing (10)
I believe the answer is:
Here is my best explanation:
'railing' is the definition.
(balustrade is a kind of railing)
'adults are moved behind black' is the wordplay.
'moved' indicates anagramming the letters.
'behind' says to put letters next to each other.
'black' becomes 'b' (as in pencil classifications).
'adultsare' with letters rearranged gives 'alustrade'.
'alustrade' put after 'b' is 'BALUSTRADE'.
(Other definitions for balustrade that I've seen before include "Railing at the side of a balcony", "form", "Parapet", "Row of short pillars with coping", "Stair rail and its supports", "Coping", "Railing supported on pillars", "Form of railing", "Railing on stairs or balcony" and "feature of terrace, perhaps".)