Twist; quirk (4)
I believe the answer is:
kink
This clue is a double definition.
'twist' is the first definition.
(I know that twist can be written as kink)
'quirk' is the second definition.
(I have seen 'Mental quirk' mean 'kink' so perhaps 'quirk' could also mean 'kink')
(Other definitions for kink that I've seen before include "Eccentricity" , "Sharp bend (in a wire) - quirk" , "Sharp twist in wire or hair, perhaps" , "Idiosyncrasy - sharp bend" , "The whim" .)