Astound Bolt's partner by beginning to sprint backwards (4)
I believe the answer is:
stun
'astound' is the definition.
(I've seen this before)
'bolt's partner by beginning to sprint backwards' is the wordplay.
'bolt's partner' becomes 'nut' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'beginning to' says to take the initial letters.
'backwards' says the letters should be written backwards.
The first letter of 'sprint' is 's'.
'nut'+'s'='nuts'
'nuts' reversed gives 'STUN'.
(Other definitions for stun that I've seen before include "Daze, render semi-conscious" , "Stupefy - daze" , "Make senseless or dizzy with a blow" , "Surprise greatly or daze" , "Nuts (anag)" .)