Railing is in raillery! (8)
I believe the answer is:
Here is my best explanation:
'railing' is the definition.
(banister is a kind of railing)
'is in raillery' is the subsidiary indication.
'in' is an insertion indicator.
'raillery' becomes 'banter' ('banter' can be a synonym of 'raillery').
'is' put into 'banter' is 'banister'.
(Other definitions for banister that I've seen before include "Upright supporting handrail of stairs", "Staircase handrail", "Rail beside a staircase", "Stair-rail", "Support running alongside stairs", "Staircase railing", "Handrail", "Stair rail" and "Stair handrail and supports".)