Vault leaving sprinter's bottom in endless pain (4)
I believe the answer is:
arch
'vault' is the definition.
(I've seen this before)
'sprinter's bottom in endless pain' is the wordplay.
'bottom' indicates one should take the final letters.
'in' is an insertion indicator.
'endless' means to remove the last letter.
'pain' becomes 'ache' (ache is a kind of pain).
The last letter of 'sprinter' is 'r'.
'ache' with its final letter removed is 'ach'.
'r' placed within 'ach' is 'ARCH'.
'leaving' acts as a link.
(Other definitions for arch that I've seen before include "Chief: crafty" , "a piece about a foot?" , "Structural curve" , "Chief - supporter - cunning" , "Waggish" .)