Loud stag gathering upset the horse-racing world (4)
I believe the answer is:
turf
'horse-racing world' is the definition.
'turf' can be an answer for 'horse-racing' ('the turf' can refer to horseracing as a sport). I am not sure about the 'world' bit.
'loud stag gathering upset' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'stag gathering' becomes 'rut' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'upset' says the letters should be written in reverse.
'f'+'rut'='frut'
'frut' back-to-front is 'TURF'.
'the' is the link.
Can you help me to learn more?
(Other definitions for turf that I've seen before include "manor" , "Gang's territory" , "Gang territory" , "Personal territory; sod" , "Sward, grass" .)