Material left by expert (4)
I believe the answer is:
lace
'material' is the definition.
(lace is a kind of material)
'left by expert' is the wordplay.
'left by' becomes 'l' (I've seen this in other clues).
'expert' becomes 'ace' (ace can informally mean very good).
'l'+'ace'='LACE'
(Other definitions for lace that I've seen before include "Delicate fabric woven in an open web" , "Fabric made in openwork design" , "Fine open fabric" , "Famous play ''Arsenic and Old . . . .''" , "Delicate open-weave fabric" .)