Taunt jogger entering endurance race, initially (4)
I believe the answer is:
jeer
'taunt' is the definition.
(I know that jeer is a more specific form of the action taunt)
'jogger entering endurance race initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'jogger entering endurance race' is 'JEER'.
(Other definitions for jeer that I've seen before include "Mock - laugh at" , "Henry ---, Richard II's enemy, successor" , "Mock rudely" , "Laugh at with derision" , "Mock publicly" .)