Ability of a runner to finish well (5)
I believe the answer is:
skill
'ability' is the definition.
(skill is a kind of ability)
'a runner to finish well' is the wordplay.
'a runner' becomes 'ski' (something which 'runs', ie moves quickly on snow).
'to finish' indicates taking half.
'well' cut in half is 'll'.
'ski'+'ll'='SKILL'
'of' is the link.
(Other definitions for skill that I've seen before include "Expertise; art" , "Aptitude" , "Knack" , "Prowess" , "facility" .)