Jumper affected teacher's sleep (13)
I believe the answer is:
steeplechaser
'jumper' is the definition.
(person or horse)
'affected teacher's sleep' is the wordplay.
'affected' indicates anagramming the letters.
'teachers'+'sleep'='teacherssleep'
'teacherssleep' anagrammed gives 'STEEPLECHASER'.
(Other definitions for steeplechaser that I've seen before include "Horse for jump races" , "Racehorse - athlete" , "Horse in an obstacle race" , "Horse that jumps" , "Horse race participant" .)