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.
('fast' can be a synonym of 'quickly')
(Other definitions for fast that I've seen before include "Tight; quick" , "Quick; firmly fixed" , "Avoid food; rapid" , "Rapid; firm" , "Starve; quick" .)