Moose comes up eating a vegetable (4)
I believe the answer is:
kale
'vegetable' is the definition.
(kale is a kind of vegetable)
'moose comes up eating a' is the wordplay.
'moose' becomes 'elk' ('elk' can be a synonym of 'moose').
'comes up' shows that the letters should be reversed in order.
'eating' means one lot of letters goes inside another.
'elk' in reverse letter order is 'kle'.
'kle' placed around 'a' is 'KALE'.
(Other definitions for kale that I've seen before include "Crinkled-leaf cabbage" , "Crinkle-leaf cabbage" , "Curly-leafed cabbage" , "Vegetable" , "Type of cabbage" .)