Not easy to understand (5,2,3)
(Other definitions for clear as mud that I've seen before include "Completely unintelligible" , "Not at all obvious" , "Obscure" , "Taut condition" , "hard to fathom" .)
'not easy to understand' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unclear that I've seen before include "Ambiguous" , "Doubtful" , "Not definite" , "Not easy to grasp" , "Poorly described" .)