He mixes oil with small quantities of vinegar (6)
I believe the answer is:
oliver
'he' is the definition.
'mixes oil with small quantities of vinegar' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'oil' is 'oli' which is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oliver that I've seen before include "Dickensian musical" , "Master Twist" , "Fictional boy who famously asked for more" , "First name of Goldsmith, great 18th century Irish writer" , "Dickens boy who asked for more, and a musical comedy" .)