Ability of a runner to get lb50 (5)
I believe the answer is:
skill
'ability' is the definition.
(skill is a kind of ability)
'runner to get lb50' is the wordplay.
'runner' becomes 'ski' (something which 'runs', ie moves quickly on snow).
'to get' says to put letters next to each other.
'lb50' becomes 'll' (I can't explain this - if you can you should give a lot more credence to this answer).
'ski'+'ll'='SKILL'
'of a' acts as a link.
Can you help me to learn more?
(Other definitions for skill that I've seen before include "facility" , "Special ability" , "Expertise; art" , "Talent" , "Ability acquired by training" .)