Course of training upset a few but not all (5)
I believe the answer is:
epsom
'course' is the definition.
(a race course)
'of training upset a few but not all' is the wordplay.
'of training' becomes 'pe' (I am not sure about this - if you are sure you should believe this answer much more).
'upset' says the letters should be written backwards.
'a few' becomes 'some' (both can mean a small amount of something).
'but not all' means to remove the last letter.
'pe' reversed gives 'ep'.
'some' with its last letter removed is 'som'.
'ep'+'som'='EPSOM'
Can you help me to learn more?
(Other definitions for epsom that I've seen before include "Classic racecourse" , "The Oaks and Derby are run here" , "Town with racecourse" , "Home to the Derby and the Oaks" , "Derby race track" .)