Rail one's found in chaff (8)
I believe the answer is:
banister
'rail' is the definition.
(banister is a kind of rail)
'one's found in chaff' is the wordplay.
'one's' becomes 'is' (roman numeral).
'found in' means one lot of letters goes inside another.
'chaff' becomes 'banter' ('banter' can be a synonym of 'chaff').
'is' inserted within 'banter' is 'BANISTER'.
(Other definitions for banister that I've seen before include "support at hand on flight" , "Rail beside a staircase" , "Support running alongside stairs" , "Upright supporting handrail of stairs" , "Stair handrail and supports" .)