Writing material popular with king (3)
I believe the answer is:
ink
'writing material' is the definition.
(I've seen this before)
'popular with king' is the wordplay.
'popular' becomes 'in'.
'with' is a charade indicator (letters next to each other).
'king' becomes 'k'.
'in'+'k'='INK'
(Other definitions for ink that I've seen before include "Writing liquid" , "Liquid from a squid" , "Dark liquid" , "Material used in printing" , "See 9" .)