One ending life of racehorse retired by the Queen (8)
I believe the answer is:
murderer
'one ending life' is the definition.
'racehorse retired by the queen' is the wordplay.
'racehorse' becomes 'redrum' (three time winner of the Grand National).
'retired' says the letters should be written backwards.
'by' says to put letters next to each other.
'the queen' becomes 'er' (Elizabeth Regina).
'redrum' back-to-front is 'murder'.
'murder'+'er'='MURDERER'
'of' acts as a link.
(Other definitions for murderer that I've seen before include "Criminal killer" , "One commits homicide" , "Assassin" , "Deliberate unlawful killer" , "Serious criminal enters demurrer" .)