Material left in minus shape (6)
I believe the answer is:
muslin
'material' is the definition.
(muslin is a type of material)
'left in minus shape' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'shape' indicates anagramming the letters.
'minus' is an anagram of 'musin'.
'l' placed within 'musin' is 'MUSLIN'.
(Other definitions for muslin that I've seen before include "Lightweight cotton cloth" , "Cotton for veil or strainer" , "Gauze-like cloth" , "dress material" , "Lightweight cotton fabric" .)