Bench (8)
'bench' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seat that I've seen before include "A manner of sitting on horse" , "Box ---, position of advantage" , "Take one and sit" , "Settle in" , "Sitting place" .)
(Other definitions for sideline that I've seen before include "Secondary job" , "second job" , "hobby" , "Subsidiary activity" , "Remove from the centre of activity" .)