Material that may be left out (4)
I believe the answer is:
felt
'material' is the definition.
(felt is a kind of material)
'left out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'left' with letters rearranged gives 'FELT'.
'that may be' acts as a link.
(Other definitions for felt that I've seen before include "Fabric of matted wool fibres" , "checked by hand" , "investigated in touching way" , "hat-maker" , "Was affected by" .)