Lots of different things thrown on mattress (10)
I believe the answer is:
assortment
'lots of different things' is the definition.
'thrown on mattress' is the wordplay.
'thrown' indicates anagramming the letters.
'on'+'mattress'='onmattress'
'onmattress' anagrammed gives 'ASSORTMENT'.
(Other definitions for assortment that I've seen before include "Medley" , "Mixture" , "salmagundi" , "Miscellany" , "Miscellaneous collection" .)