Clear riotous thousand mobbing author (4-3-4)
I believe the answer is:
open-and-shut
'clear' is the definition.
(thesaurus)
'riotous thousand mobbing author' is the wordplay.
'riotous' indicates anagramming the letters.
'mobbing' means one lot of letters goes inside another (mob can mean to surround or crowd round).
'author' becomes 'pen' (to pen something is to author it).
'thousand' anagrammed gives 'oandshut'.
'oandshut' enclosing 'pen' is 'OPEN-AND-SHUT'.
(Other definitions for open-and-shut that I've seen before include "No nude paths (anag) - easily decided" , "So obvious as to be easily decided" , "Straightforward" .)