Bed or stratum (5)
I believe the answer is:
layer
This clue is a double definition.
'bed' is the first definition.
(I know that bed can be written as layer)
'stratum' is the second definition.
(I've seen this in another clue)
(Other definitions for layer that I've seen before include "One supplying eggs" , "Source of eggs" , "Film" , "One of several thicknesses" , "Early (anag.)" .)