Not liable to fade quickly (4)
I believe the answer is:
fast
'quickly' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fast that I've seen before include "Unable to move" , "Avoid food; rapid" , "Speedily - secured" , "requirement before surgery" , "debauched" .)