Old agricultural worker upsets Charlie starting oil painting (6)
I believe the answer is:
fresco
'painting' is the definition.
(fresco is a kind of painting)
'old agricultural worker upsets charlie starting oil' is the wordplay.
'old agricultural worker' becomes 'serf' (I can't justify this - if you can you should give a lot more credence to this answer).
'upsets' is a reversal indicator.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'starting' says to take the initial letters.
The first letter of 'oil' is 'o'.
'serf' backwards is 'fres'.
'fres'+'c'+'o'='FRESCO'
Can you help me to learn more?
(Other definitions for fresco that I've seen before include "Mode of wall painting" , "Painting style" , "Mural painted on a wall" , "Forces (anag) - wall painting" , "Method of painting on walls" .)