A legal bet placed on a mere trifle (9)
I believe the answer is:
bagatelle
'a mere trifle' is the definition.
'a legal bet placed' is the wordplay.
'placed' indicates anagramming the letters (placing the letters).
'a'+'legal'+'bet'='alegalbet'
'alegalbet' with letters rearranged gives 'BAGATELLE'.
'on' is the link.
(Other definitions for bagatelle that I've seen before include "Trivial matter" , "lovely trinket" , "shooting game" , "Something trivial" , "musical composition" .)