Extremist cultivated his veg (6)
I believe the answer is:
radish
'veg' is the definition.
(radish is a type of vegetable)
'extremist cultivated his' is the wordplay.
'extremist' becomes 'rad' (I've seen this in another clue).
'cultivated' is an anagram indicator.
'his' is an anagram of 'ish'.
'rad'+'ish'='RADISH'
(Other definitions for radish that I've seen before include "Red cold salad ingredient" , "Root vegetable used in salads" , "Hard vegetable" , "Daikon or mooli" , "Plant with crisp root eaten raw" .)