Containers that slate faces not even filled (7)
I believe the answer is:
troughs
'containers' is the definition.
(trough is a kind of container)
'that slate faces not even filled' is the wordplay.
'faces' says to take the initial letters (I've seen 'face' mean this).
'not even' becomes 'rough'.
'filled' indicates putting letters inside.
The first letters of 'that slate' is 'ts'.
'ts' going around 'rough' is 'TROUGHS'.
(Other definitions for troughs that I've seen before include "Containers for feeding cattle and horses" , "Hollows between waves" , "Depressions" , "Long narrow receptacles with food or water for animals" , "Channels; low points" .)