Staircase adjunct (8)
I believe the answer is:
'staircase adjunct' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for banister that I've seen before include "Stair-rail" , "Staircase railing" , "Staircase handrail" , "Rail beside a staircase" , "Stair rail" .)