Rail is straddled by kid (8)
I believe the answer is:
banister
'rail' is the definition.
(banister is a kind of rail)
'is straddled by kid' is the wordplay.
'straddled by' is an insertion indicator.
'kid' becomes 'banter' ('banter' can be a synonym of 'kid').
'is' placed into 'banter' is 'BANISTER'.
(Other definitions for banister that I've seen before include "Staircase handrail" , "Support running alongside stairs" , "Rail beside a staircase" , "Stair-rail" , "Staircase railing" .)