Man, say, with working material (5)
I believe the answer is:
rayon
'material' is the definition.
(rayon is a type of fabric)
'man say with working' is the wordplay.
'man say' becomes 'Ray' (Man Ray was a 20th-century artist).
'with' is a charade indicator (letters next to each other).
'working' becomes 'on' (e.g. a machine is working or on).
'ray'+'on'='RAYON'
(Other definitions for rayon that I've seen before include "Artificial silk fabric" , "Viscose fibre" , "Man-made fibre" , "Man-made fabric" , "Artificial fibre" .)