Difficult to understand (4)
'difficult to understand' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for deep that I've seen before include "dangerous for swimming?" , "hard to understand" , "Profound - cunning - sea - bass" , "Bottomless" , "Intense; low (in pitch)" .)
(Other definitions for clear as mud that I've seen before include "Completely unintelligible" , "Obscure" , "Taut condition" , "Not at all obvious" , "hard to fathom" .)