Experimental equipment for examination underground (4,4)
I believe the answer is:
test tube
'experimental equipment' is the definition.
'examination underground' is the wordplay.
'examination' becomes 'test' ('test' can be a synonym of 'examination').
'underground' becomes 'tube' ('tube' can be a synonym of 'underground').
'test'+'tube'='TEST-TUBE'
'for' is the link.
(Other definitions for test tube that I've seen before include "Item of laboratory equipment" , "Laboratory instrument" , "glass vessel" , "Hollow cylinder for the laboratory" , "Ling glass cylinder used in laboratories" .)