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 a task' becomes 'sk' (I can't explain this - if you can you should give a lot more credence to this answer).
'badly?' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'sk'+'ill'='SKILL'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for skill that I've seen before include "Ability acquired by training" , "Ability - dexterity" , "Expertise; art" , "Prowess" , "Expertise, dexterity" .)