Check out underground laboratory item (4,4)
I believe the answer is:
test tube
'laboratory item' is the definition.
(I've seen this before)
'check out underground' is the wordplay.
'check out' becomes 'test' (I've seen this in another clue).
'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 "where reactions are observed" , "Lab vessel" , "Factor in experimental procedure" , "experimental gear?" , "Hollow cylinder for the laboratory" .)