Writing material on king revealing mental quirk (4)
I believe the answer is:
kink
'mental quirk' is the definition.
(I've seen this before)
'writing material on king' is the wordplay.
'writing material' becomes 'ink' (substance for writing with).
'on' says to put letters next to each other.
'king' becomes 'k' (chess abbreviation).
'ink' put after 'k' is 'KINK'.
'revealing' acts as a link.
(Other definitions for kink that I've seen before include "Bend or twist" , "Sharp bend (in a wire) - quirk" , "Quirk; imperfection" , "Constricting bend in pipe" , "Quirk of behaviour" .)