Ordinary brook by A-road (8)
I believe the answer is:
standard
'ordinary' is the definition.
(thesaurus)
'brook by a-road' is the wordplay.
'brook' becomes 'stand' ('stand' can be a synonym of 'brook'**).
'by' is a charade indicator (letters next to each other).
'road' becomes 'rd' (abbreviation).
'stand'+'a'+'rd'='STANDARD'
(Other definitions for standard that I've seen before include "possibly rose" , "'Reference point, touchstone (8)'" , "Ceremonial flag" , "Against which to measure the rest" , "some soldiers find bearable?" .)