Ability to finish a task badly? (5)
I believe the answer is:
skill
'ability' is the definition.
(skill is a kind of ability)
'finish a task badly?' is the wordplay.
'finish' suggests halving the letters.
'badly?' becomes 'ill' (eg in ill-advised).
'task' cut in half is 'sk'.
'sk'+'ill'='SKILL'
'to' is the link.
(Other definitions for skill that I've seen before include "Ability acquired by training" , "Ability - dexterity" , "Expertise; art" , "Prowess" , "Expertise, dexterity" .)