Popular king's writing material (3)
I believe the answer is:
ink
'writing material' is the definition.
(I've seen this in another clue)
'popular king's' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'king' becomes 'K' (abbreviation for king in chess).
'in'+'k'='INK'
(Other definitions for ink that I've seen before include "Writing liquid" , "Fluid from cuttlefish" , "Dark liquid" , "Writing fluid" , "form of writing" .)