King longing to restrict son’s trashy material (6)
I believe the answer is:
kitsch
'trashy material' is the definition.
(kitschy items)
'king longing to restrict son's' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'longing' becomes 'itch' (I've seen this in other clues).
'to restrict' indicates putting letters inside (restrict can mean pen in or contain).
'itch' placed around 's' is 'itsch'.
'k'+'itsch'='KITSCH'
(Other definitions for kitsch that I've seen before include "Sentimental or pretentious "art"" , "Item of pretentious bad taste" , "Vulgar, sentimental, art" , "Excessively garish or sentimental" , "Garish and in bad taste" .)