Fix furrow up for vegetable (6)
I believe the answer is:
turnip
'for vegetable' is the definition.
(turnip is a kind of vegetable)
'fix furrow up' is the wordplay.
'fix' becomes 'pin' (to pin is to fix in place).
'furrow' becomes 'rut' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'pin'+'rut'='pinrut'
'pinrut' written backwards gives 'TURNIP'.
(Other definitions for turnip that I've seen before include "'Root vegetable, yellow or white (6)'" , "food item" , "Large-rooted vegetable related to the cabbage" , "veggie" , "Plant with round white root used as vegetable" .)