Jeering now and then, loud fans riot -- case overturned (8)
I believe the answer is:
scornful
'jeering' is the definition.
'now and then loud fans riot case overturned' is the wordplay.
'now and then' means one should take alternating letters (letters taken on a regular basis, now and then).
'overturned' says the letters should be written in reverse.
'loud'+'fans'+'riot'+'case'='loudfansriotcase'
The alternate letters of 'loudfansriotcase' are 'lufnrocs'.
'lufnrocs' reversed gives 'SCORNFUL'.
(Other definitions for scornful that I've seen before include "Dismissive, contemptuous" , "Openly contemptuous" , "Showing contempt for" , "Contemptuous or derisory" , "Disdainful, contemptuous" .)