Meeting a variable cost (5)
I believe the answer is:
ascot
'meeting' is the definition.
(horse racing meeting)
'a variable cost' is the wordplay.
'variable' is an anagram indicator (a varied form of the letters).
'cost' with letters rearranged gives 'scot'.
'a'+'scot'='ASCOT'
(Other definitions for ascot that I've seen before include "Berkshire town, famous for horse racing" , "Berks racecourse" , "Tie or cravat" , "Horse-racing venue" , "Tosca (anag) - racecourse" .)