Like lightning — don't eat (4)
I believe the answer is:
fast
I believe this clue is a double definition.
'like lightning' is the first definition.
(I've seen this in another clue)
'don't eat' is the second definition.
(fasting is not eating)
(Other definitions for fast that I've seen before include "Firmly attached; rapid" , "Lent, perhaps" , "Period during which fare is reduced" , "Quickly; tight" , "not lento" .)