Expertise in painting (3)
I believe the answer is:
art
This clue is a double definition.
'expertise' is the first definition.
(I've seen this before)
'painting' is the second definition.
(painting is an art)
'in' acts as a link.
(Other definitions for art that I've seen before include "crafty conduct" , "Painting, for instance" , "Drawing" , "Creative works; knack" , "eg Painting" .)