Vague old poet caught while circulating slander (5,2,3)
I believe the answer is:
clear as mud
'vague' is the definition.
(thesaurus)
'old poet caught while circulating slander' is the wordplay.
'old poet' becomes 'lear' (Victorian poet Edward Lear).
'caught' becomes 'c' (cricket abbreviation).
'while' becomes 'as'.
'circulating' means one lot of letters goes inside another (to circulate is to go round).
'slander' becomes 'mud' (as in to drag someone's name through the mud).
'c'+'as'='cas'
'lear' inserted into 'cas' is 'clearas'.
'clearas'+'mud'='CLEAR AS MUD'
(Other definitions for clear as mud that I've seen before include "Taut condition" , "Obscure" , "hard to fathom" , "Completely unintelligible" , "Not at all obvious" .)