The “rail” is in “raillery” (8)
I believe the answer is:
banister
'the rail' is the definition.
(banister is a kind of rail)
'is in raillery' is the wordplay.
'in' indicates putting letters inside.
'raillery' becomes 'banter' (synonyms).
'is' put within 'banter' is 'BANISTER'.
(Other definitions for banister that I've seen before include "Stair-rail" , "Staircase railing" , "Support running alongside stairs" , "support at hand on flight" , "Upright supporting handrail of stairs" .)