One way of working that includes bygone painting technique (7)
I believe the answer is:
impasto
'painting technique' is the definition.
(I've seen this before)
'one way of working that includes bygone' is the wordplay.
'one' becomes 'i' (Roman numeral).
'way of working' becomes 'mo' (I've seen this in other clues).
'that includes' means one lot of letters goes inside another.
'bygone' becomes 'past' ('past' can be similar in meaning to 'bygone').
'mo' going around 'past' is 'mpasto'.
'i'+'mpasto'='IMPASTO'
(Other definitions for impasto that I've seen before include "application of paint in this way" , "Technique of laying on paint thickly so that it stands out from a surface" , "Slapping paint on" , "Thickly-applied painting technique" , "Thick coat required" .)