Special skill (9)
(Other definitions for expertise that I've seen before include "Special skill or knowledge" , "Craft" , "professionalism" , "Technical skill" , "Mastery, proficiency" .)
'special skill' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knack that I've seen before include "Skill in doing something" , "Slope" , "Ability, facility" , "Technical aptitude" , "Skill for specific task" .)