1000 unite riot in a trice (6)
I believe the answer is:
minute
'a trice' is the definition.
(thesaurus)
'1000 unite riot in' is the wordplay.
'm000' becomes 'm' ('m' can be a synonym of '1000'**).
'riot' indicates an anagram.
'in' means one lot of letters goes inside another.
'unite' with letters rearranged gives 'inute'.
'm' put within 'inute' is 'MINUTE'.
(Other definitions for minute that I've seen before include "Precise" , "Tiny; written note" , "60s" , "official memorandum" , "Measure of time" .)