Lab equipment used to examine underground system? (4,4)
I believe the answer is:
test tube
'lab equipment' is the definition.
(I've seen this before)
'to examine underground system?' is the wordplay.
I cannot really understand how this works, but
'examine' could be 'test' (synonyms) and 'test' is found in the answer.
'underground' could be 'tube' (the London Underground is nicknamed the Tube) and 'tube' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'used' is the link.
Can you help me to learn more?
(Other definitions for test tube that I've seen before include "Laboratory experimental container" , "Laboratory glassware" , "Hollow cylinder for the laboratory" , "scientific birthplace" , "Lab equipment" .)