MP backed horse securing lead in Derby (6)
I believe the answer is:
redcap
'mp' is the definition.
(redcap is a kind of mp)
'backed horse securing lead in derby' is the wordplay.
'backed' shows that the letters should be reversed in order.
'horse' becomes 'pacer' (pacer is a kind of horse**).
'securing' indicates putting letters inside.
'lead in' suggests taking the first letters.
The first letter of 'derby' is 'd'.
'pacer' going around 'd' is 'pacder'.
'pacder' written backwards gives 'REDCAP'.
(Other definitions for redcap that I've seen before include "bird!" , "Hole from explosion" , "Goldfinch" , "MP" , "Military policeman (slang)" .)