They're round holes, of course, in vegetables (6)
I believe the answer is:
greens
'vegetables' is the definition.
(green is a kind of vegetable)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for greens that I've seen before include "party politicians" , "Leaves" , "Leafy vegetables" , "They're for putting on - vegetables" , "Vegetables and politicians of a certain hue" .)