Weaken, slowing down following a series of races (6)
I believe the answer is:
attrit
'weaken' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'slowing down following a series of races' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'races' could be 'tt' (Isle of Man Tourist Trophy races) and 'tt' is located in the answer.
'slowing down' could be 'rit' (abbreviation for musical term ritardando) and 'rit' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for attrit that I've seen before include "Wearing down" , "forceful dispute" , "Destroy" .)