Dark liquid that could be cloudier (5,3)
I believe the answer is:
crude oil
'dark liquid' is the definition.
(crude oil is dark in colour)
'could be cloudier' is the wordplay.
'could be' indicates an anagram.
'cloudier' with letters rearranged gives 'CRUDE OIL'.
'that' acts as a link.
(Other definitions for crude oil that I've seen before include "liquid asset" , "Source of fuel" , "output from rig" , "that has boring producers?" , "From which petrol is made" .)