Thick fluid but so clear, essentially (6)
I believe the answer is:
obtuse
'thick' is the definition.
(I've seen this before)
'fluid but so clear essentially' is the wordplay.
'fluid' is an anagram indicator.
'essentially' indicates the central letters.
The middle letter of 'clear' is 'e'.
'but'+'so'='butso'
'butso' with letters rearranged gives 'obtus'.
'obtus'+'e'='OBTUSE'
(Other definitions for obtuse that I've seen before include "Dense, dim about the angle" , "not acute?" , "Slow, dull" , "Orchestra; stripe" , "Not very perceptive or sensitive" .)