The knack of painting (3)
I believe the answer is:
art
I believe this is a double definition.
'the knack' is the first definition.
(I've seen this before)
'painting' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for art that I've seen before include "Eg, paintings, sculpture" , "The works of Alma-Tadema" , "Beautiful or significant products of human creativity" , "For example, Blakey's creative" , "Creative works; knack" .)