Stratum or seam (5)
I believe the answer is:
layer
I believe this clue is a double definition.
'stratum' is the first definition.
(I've seen this before)
'seam' is the second definition.
(I've seen this before)
(Other definitions for layer that I've seen before include "Single thickness, tier" , "Single thickness and hen" , "Coating" , "Early (anag.)" , "course" .)