Must cut length in light cloth (6)
I believe the answer is:
muslin
'light cloth' is the definition.
(I've seen this before)
'must cut length in' is the wordplay.
'cut' means to remove the last letter.
'length' becomes 'l' (abbreviation in geometry).
'must' with its last letter removed is 'mus'.
'mus'+'l'+'in'='MUSLIN'
(Other definitions for muslin that I've seen before include "Light cotton cloth" , "Lightweight cotton fabric" , "Lightweight cotton cloth" , "Gauze-like cloth" , "Fine soft plain-woven cotton fabric of gauzy appearance" .)