Slave put up first of Caravaggios, old painting on wall (6)
I believe the answer is:
'painting on wall' is the definition.
'fresco' can be an answer for 'wall' (a fresco is a painting done on a wall). I'm not sure about the 'painting on' bit.
'slave put up first of caravaggios old' is the wordplay.
'slave' becomes 'serf' (I've seen this in another clue).
'put up' says the letters should be written in reverse.
'first of caravaggios' becomes 'c' (1st letter of 'caravaggios').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'serf' backwards is 'fres'.
Can you help me to learn more?
(Other definitions for fresco that I've seen before include "Artistic work" , "Mural painting" , "Forces (anag) - wall painting" , "Painting using watercolours on plaster" , "Painting style" .)