Mixed bag up, prior to mixing (9)
I believe the answer is:
potpourri
'mixed bag' is the definition.
(I've seen this before)
'up prior to mixing' is the wordplay.
'mixing' is an anagram indicator.
'up'+'prior'+'to'='uppriorto'
'uppriorto' is an anagram of 'POTPOURRI'.
(Other definitions for potpourri that I've seen before include "Pleasant-smelling mixture" , "Mixture (of flower petals and spices?)" , "Petals and spices to perfume a room" , "Dried petal mix" , "Dried petals for scent" .)