River transport laboratory item (4,4)
I believe the answer is:
test tube
'laboratory item' is the definition.
(I've seen this before)
'river transport' is the wordplay.
'river' becomes 'test' (the River Test).
'transport' becomes 'tube' (the London Underground is nicknamed the Tube).
'test'+'tube'='TEST-TUBE'
(Other definitions for test tube that I've seen before include "Lab equipment" , "Glass item in lab" , "Experimental glassware" , "birthplace of science" , "scientific birthplace" .)