The opposite of fast? (7)
I believe the answer is:
overeat
'the opposite of fast?' is the definition.
(fast can mean to avoid eating food)
This is the entire clue.
(Other definitions for overeat that I've seen before include "Gobble to excess" , "Gorge oneself" , "Binge" , "Take in excess" , "Indulge in gluttony" .)