Not liable to run quickly (4)
I believe the answer is:
fast
'quickly' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fast that I've seen before include "debauched" , "Avoid food; rapid" , "Brisk" , "Period during which fare is reduced" , "Moored - fleet" .)