Fastest runner caught — he can't stand up (7)
I believe the answer is:
cheetah
'fastest runner' is the definition.
(fastest land animal)
'caught he can't stand up' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'can't stand' becomes 'hate'.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'hate' backwards is 'etah'.
'c'+'he'+'etah'='CHEETAH'
(Other definitions for cheetah that I've seen before include "Fast-running creature" , "Feline -- fastest land animal" , "Cat renowned for speed" , "Fast-moving creature" , "Spotted creature" .)