Don't eat quickly (4)
I believe the answer is:
This clue is a double definition.
'don't eat' is the first definition.
(fasting is not eating)
'quickly' is the second definition.
(I know that quickly can be written as fast)
(Other definitions for fast that I've seen before include "Tight; quick" , "Quick; firmly fixed" , "Avoid food; rapid" , "Rapid; firm" , "Starve; quick" .)