Insubstantial meal, mostly greens, half finished (6)
I believe the answer is:
meagre
'insubstantial' is the definition.
(I've seen this before)
'meal mostly greens half finished' is the wordplay.
'mostly' means to remove the last letter.
'half finished' indicates taking half.
'meal' with its final letter taken away is 'mea'.
'greens' cut in half is 'gre'.
'mea'+'gre'='MEAGRE'
(Other definitions for meagre that I've seen before include "Lacking in quantity or quality" , "lacking in strength" , "Thin or scanty" , "Polite form of address" , "Scanty, skimpy" .)