Foremost hybrid roses developed by those in Derby (6)
I believe the answer is:
horses
'foremost hybrid roses developed by those in derby' is the definition.
I can't judge whether this definition defines the answer.
'foremost hybrid roses developed by' is the wordplay.
'foremost hybrid' becomes 'h' (I am not sure about this - if you are sure you should believe this answer much more).
'developed' indicates anagramming the letters.
'by' says to put letters next to each other.
'roses' anagrammed gives 'orses'.
'h'+'orses'='HORSES'
Can you help me to learn more?
(Other definitions for horses that I've seen before include "Domesticated animals" , "Hoofed mammals (which go with 19 4)" , "Eg, stallions" , "Equine beasts" , "Equines" .)