Starter of organic root vegetable turned over in sauce (4)
I believe the answer is:
mayo
'sauce' is the definition.
'starter of organic root vegetable turned over' is the wordplay.
'starter of' suggests taking the first letters.
'root vegetable' becomes 'yam' (yam is a kind of root vegetable).
'turned over' says the letters should be written backwards.
The first letter of 'organic' is 'o'.
'o'+'yam'='oyam'
'oyam' in reverse letter order is 'MAYO'.
'in' is the link.
(Other definitions for mayo that I've seen before include "County of north-west Ireland" , "Irish county; dressing" , "Thick creamy dressing" , "Cold, creamy sauce (abbr)" , "Ir. county; salad dressing" .)