Try out underground piece of laboratory apparatus (4,4)
I believe the answer is:
test tube
'piece of laboratory apparatus' is the definition.
'try out underground' is the wordplay.
'try out' becomes 'test' ('test' can be a synonym of 'try out').
'underground' becomes 'tube' ('tube' can be a synonym of 'underground').
'test'+'tube'='TEST-TUBE'
(Other definitions for test tube that I've seen before include "Hollow cylinder for the laboratory" , "Lab equipment" , "Lab vessel" , "useful for experiment" , "scientific birthplace" .)