Sportsman's warning over athlete's predecessor (10)
I believe the answer is:
forerunner
'predecessor' is the definition.
(I know that predecessor can be written as forerunner)
'sportsman's warning over athlete's' is the wordplay.
'sportsman's warning' becomes 'fore' (I can't justify this - if you can you should give a lot more credence to this answer).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'athlete' becomes 'runner' (type of athlete).
'fore'+'runner'='FORERUNNER'
Can you help me to learn more?
(Other definitions for forerunner that I've seen before include "Precursor" , "one coming ahead" .)