Fabric must largely zip up (6)
I believe the answer is:
muslin
'fabric' is the definition.
(I know that muslin is a type of fabric)
'must largely zip up' is the wordplay.
'largely' means to remove the last letter (most but not all of the letters).
'zip' becomes 'nil' (both can mean zero).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'nil' in reverse letter order is 'lin'.
'must' with its final letter taken away is 'mus'.
'mus'+'lin'='MUSLIN'
(Other definitions for muslin that I've seen before include "dress material" , "Lightweight cotton fabric" , "Gauze-like cloth" , "Light cotton cloth" , "Fine soft plain-woven cotton fabric of gauzy appearance" .)