Railing is in raillery! (8)
I believe the answer is:
Here is my best explanation:
'railing' is the definition.
(I've seen this before)
'is in raillery' is the wordplay.
'in' means one lot of letters goes inside another.
'raillery' becomes 'banter' ('banter' can be a synonym of 'raillery').
'is' put inside 'banter' is 'BANISTER'.
(Other definitions for banister that I've seen before include "Handrail", "Rail beside a staircase", "Stair rail", "Railing", "Upright supporting handrail of stairs", "Staircase railing", "Support running alongside stairs", "Stair-rail", "Staircase handrail" and "support at hand on flight".)