Champion runner an adulterer, they say (7)
I believe the answer is:
cheetah
'champion runner' is the definition.
(fastest land animal)
'an adulterer they say' is the wordplay.
'an adulterer' becomes 'cheater' (someone cheating on their partner).
'they say' indicates a 'sounds like' (homophone) clue.
'cheater' is a homophone of 'CHEETAH'.
(Other definitions for cheetah that I've seen before include "Swift predator" , "Swift large wild cat with spotted coat" , "16 21 [INDIAN IMPORT]" , "Feline, the fastest mammal" , "Fast predator" .)