Laboratory glass for experiment underground? (4,4)
I believe the answer is:
test tube
'laboratory glass' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'experiment underground?' is the wordplay.
'experiment' becomes 'test' (test is a kind of experiment).
'underground?' becomes 'tube' (the London Underground is nicknamed the Tube).
'test'+'tube'='TEST-TUBE'
'for' is the link.
Can you help me to learn more?
(Other definitions for test tube that I've seen before include "Small glass cylinder with one end closed, used in laboratory" , "Ling glass cylinder used in laboratories" , "item of scientific use?" , "Factor in experimental procedure" , "Lab vessel" .)