Clue: Loud stag gathering upset the horse-racing world (4)
Answer: TURF
Explanation: Definition: horse-racing world
 **Reversal of**(… upset, in a down clue) [ F(abbrev. for "forte", a musical direction to play loudly) + RUT(a stag gathering, or more precisely, the time of year when stags gather to fight each other to decide who mates with whom) ].
Defn: A general term for horse-racing and what goes with it.