Extreme, his new salad ingredient (6)
I believe the answer is:
radish
'salad ingredient' is the definition.
(radishes are often used in salads)
'extreme his new' is the wordplay.
'extreme' becomes 'rad' (I've seen this in another clue).
'new' indicates anagramming the letters.
'his' with letters rearranged gives 'ish'.
'rad'+'ish'='RADISH'
(Other definitions for radish that I've seen before include "Root vegetable used in salads" , "Pungent red salad root" , "Edible small root" , "Sir had this vegetable" , "Red cold salad ingredient" .)