Test site
'test site' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lab that I've seen before include "Socialist" , "Science room" , "Scientist's workroom" , "biology classroom?" , "Place for experiments" .)
(Other definitions for class that I've seen before include "Category; form" , "People having the same social or economic status" , "Genus" , "School group" , "sort" .)