Try out underground lab equipment (4,4)
I believe the answer is:
test tube
'lab equipment' is the definition.
(I've seen this before)
'try out underground' is the wordplay.
'try out' becomes 'test' (synonyms).
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'test'+'tube'='TEST-TUBE'
(Other definitions for test tube that I've seen before include "Cylindrical glass in a laboratory" , "Laboratory item (for babies?)" , "scientific birthplace" , "Container for precipitation" , "Lab vessel" .)