Glass cylinder to examine on the Underground (4,4)
I believe the answer is:
test tube
'glass cylinder' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'examine on the underground' is the wordplay.
'examine' becomes 'test' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'test'+'tube'='TEST-TUBE'
'to' is the link.
Can you help me to learn more?
(Other definitions for test tube that I've seen before include "Lab vessel" , "'Laboratory glass cylinder (4,4)'" , "Lab equipment" , "item of scientific use?" , "glass vessel" .)