Court artist with a twitch and an equation (9)
I believe the answer is:
quadratic
'an equation' is the definition.
(I know that quadratic is a type of equation)
'court artist with a twitch' is the wordplay.
'court' becomes 'quad' (quad can mean a rectangular courtyard).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'with' means one lot of letters go next to another.
'a twitch' becomes 'tic' (tic is a kind of twitch).
'quad'+'ra'+'tic'='QUADRATIC'
'and' acts as a link.
(Other definitions for quadratic that I've seen before include "Second-power equation" , "Type of algebraic equation" , "(In algebra) kind of equation - resembling a square" , "Sort of equation in algebra" , "(In maths) a type of equation" .)