Dispatch old racehorse from the East (6)
I believe the answer is:
murder
'dispatch' is the definition.
(I know that dispatch can be written as murder)
'old racehorse from the east' is the wordplay.
'old racehorse' becomes 'red rum' (I've seen this before).
'from the east' says the letters should be written in reverse (on a map from right to left).
'redrum' back-to-front is 'MURDER'.
(Other definitions for murder that I've seen before include "Homicide (very noisy when blue)" , "Voraciously consume" , "a lot of crows" , "hit" , "Unlawful premeditated killing" .)