Sleep interrupted by king is rubbish (5)
I believe the answer is:
dross
'is rubbish' is the definition.
(I've seen this before)
'sleep interrupted by king' is the wordplay.
'sleep' becomes 'doss' (to doss is to sleep in an improvised bed).
'interrupted by' is an insertion indicator.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'doss' going around 'r' is 'DROSS'.
(Other definitions for dross that I've seen before include "Worthless or dangerous material" , "Waste from molten metal" , "Lees" , "Something of inferior quality" , "Waste coal - rubbish" .)