In the tunnel cats bounce, going over the barrier (8)
I believe the answer is:
obstacle
'the barrier' is the definition.
(I've seen this before)
'in the tunnel cats bounce going over' is the wordplay.
'in' says the answer is hidden in the clue.
'going over' says the letters should be written in reverse.
'elcatsbo' can be found hidden inside 'tunnel cats bounce'.
'elcatsbo' in reverse letter order is 'OBSTACLE'.
(Other definitions for obstacle that I've seen before include "Hindrance, obstruction" , "race" , "It hinders or prevents advance" , "A spanner in the works?" , "Hurdle, impediment" .)