The most common and most likely answer to this crossword clue is the 7 letter word CHEETAH
I've seen this crossword clue in The Times
C
H
E
E
T
A
H
More clues leading to the result CHEETAH
- Fast runner no sporting type we're told
- Fastest runner caught — he can't stand up
- Teacher briefly trained house's first champion runner
- Renowned runner of EC getting involved with Heath
- Champion runner an adulterer, they say
- One spotted adulterer in conversation
- Hear 'rat' for 'cat'
- Revolutionary recalls loathing for large cat
- Blast covering the ground fast, the ache getting worse
- Report of rogue animal
Buddy explains!

The straight part
We know that "Fast runner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fast runner = cheetah.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CHEETAH mean?
CHEETAH noun- A distinctive member (Acinonyx jubatus) of the cat family, slightly smaller than the leopard, but with proportionately longer limbs and a smaller head. It is native to Africa and also credited with being the fastest terrestrial animal.