Cryptic caves

A hint as this isn't a cave ending in the normal pot, cave, swallet, etc.
First word describes building material such as concrete or brick that has a vitrified surface (also the word is a synonym for something of poor quality). The second word is a synonym - think base of a valley, or the part of your body that you sit on! Horribly cryptic and not the most well known of caves.
 
The reference to concrete threw me. As a civil engineer, I've never heard of concrete with a vitrified surface (brick, yes), but clinker is an intermediate step in the manufacture of cement, which is one constituent of concrete.
 
Ok, try this important but rarely visited site.....

"Factory in the clear given reorganisation?" ( 6,4,4 )
 
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