Mostly trim, then cut, root vegetable (7)
I believe the answer is:
parsnip
'root vegetable' is the definition.
(I know that parsnip is a type of root vegetable)
'mostly trim then cut' is the wordplay.
'mostly' means to remove the last letter.
'trim' becomes 'pare' (synonyms).
'then' is a charade indicator (letters next to each other).
'cut' becomes 'snip' (snip is a kind of cut).
'pare' with its last letter taken off is 'par'.
'par'+'snip'='PARSNIP'
(Other definitions for parsnip that I've seen before include "Yellow root vegetable" , "Type of root veg" , "Sweet root vegetable" , "food" , "A white root vegetable" .)