Fast runner no sporting sort we're told (7)
I believe the answer is:
cheetah
'fast runner' is the definition.
(I've seen this before)
'no sporting sort we're told' is the wordplay.
'no sporting sort' becomes 'cheater' (eg someone cheating in sport).
'we're told' indicates a 'sounds like' (homophone) clue.
'cheater' is a homophone of 'CHEETAH'.
(Other definitions for cheetah that I've seen before include "Member of cat family" , "Long-legged spotted big cat" , "Swift, spotted feline" , "Fast-running creature" , "Fastest cat" .)