Don't eat so quickly (4)
I believe the answer is:
fast
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)
'so' is the link.
(Other definitions for fast that I've seen before include "At high speed" , "Moored - fleet" , "eg Ramadan; speedy" , "Starve; quick" , "Securely fixed" .)