Adults are moved behind black railing (10)
I believe the answer is:
balustrade
'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).
'adults'+'are'='adultsare'
'adultsare' with letters rearranged gives 'alustrade'.
'alustrade' put after 'b' is 'BALUSTRADE'.
(Other definitions for balustrade that I've seen before include "Railing on stairs or balcony" , "Stair rail and its supports" , "Parapet" , "Railing supported on pillars" , "Row of short pillars with coping" .)