Smashing on mattress, showing variety! (10)
I believe the answer is:
assortment
'variety' is the definition.
('assortment' can be a synonym of 'variety')
'smashing on mattress' is the wordplay.
'smashing' indicates anagramming the letters.
'on'+'mattress'='onmattress'
'onmattress' anagrammed gives 'ASSORTMENT'.
'showing' is the link.
(Other definitions for assortment that I've seen before include "salmagundi" , "Medley" , "Miscellaneous collection" , "Mixture" , "Variety" .)