"Dig in!” (3)
'dig in' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eat that I've seen before include "Scoff" , "Be corrosive" , "Worry" , "Bite, chew and swallow" , "Break bread" .)
(Other definitions for eat up that I've seen before include "Consume completely" , "43 [TUCK] in" , "Empty plate" , "Exhaust" , "Finish course" .)