Science equipment to examine underground system (4,4)
I believe the answer is:
test tube
'science equipment' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'examine underground system' is the wordplay.
I cannot really understand how this works, but
'examine' could be 'test' (testing is a kind of examining) and 'test' is found within the answer.
'underground' could be 'tube' (the London Underground is nicknamed the Tube) and 'tube' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for test tube that I've seen before include "where reactions are observed" , "birthplace of science" , "useful for experiment" , "Factor in experimental procedure" , "Thin glass receptacle used in laboratory" .)