### Clue: Index of success in game (7)
### Answer: FOOTSIE 
 ### Explanation: Definition: Index
 S(abbrev. for "success", to be found as part of an acronym, but I can't find a reference to a stand-alone abbrev.) **contained in**(in) FOOTIE(or footy, an informal name for the game of football).
Answer: A nickname for the Financial Times Stock Exchange 100 Index.