Gold cat with a twitch that's involuntary (9)
I believe the answer is:
automatic
'involuntary' is the definition.
('automatic' can be similar in meaning to 'involuntary')
'gold cat with a twitch' is the wordplay.
'gold' becomes 'au' ('Au' can be a synonym of 'gold').
'cat' becomes 'tom' (a tom is a male cat).
'with' says to put letters next to each other.
'twitch' becomes 'tic' (tic is a kind of twitch).
'au'+'tom'+'a'+'tic'='AUTOMATIC'
'that's' is the link.
(Other definitions for automatic that I've seen before include "Instinctive or mechanical" , "Without volition or conscious control" , "Done without thinking" , "Occurring as a matter of course" , "washing-machine?" .)