Eat no food in banquet that lacks energy (4)
I believe the answer is:
fast
'eat no food' is the definition.
'banquet that lacks energy' is the wordplay.
'banquet' becomes 'feast' ('feast' can be a synonym of 'banquet').
'that lacks' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'feast' with 'e' taken out is 'FAST'.
'in' acts as a link.
(Other definitions for fast that I've seen before include "firm to > go without" , "Rapidly -- dissipated" , "Quick - don't eat" , "Avoid food; rapid" , "Moving quickly" .)