Physical substance (6)
I believe the answer is:
matter
'physical substance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for matter that I've seen before include "Issue - stuff" , "Carry weight" , "No . . . . . . - it is unimportant" , "It occupies space, unlike mind or spirit" , "See 10" .)