Dickens's old fool (4)
I believe the answer is:
bozo
'fool' is the definition.
(bozo can mean a foolish person)
'dickens's old' is the wordplay.
'dickens' becomes 'boz' (I've seen this in another clue).
'old' becomes 'o' (abbreviation).
'boz'+'o'='BOZO'
(Other definitions for bozo that I've seen before include "Clot" , "American chap" , "Dimwit" , "twit" , "Stupid man (US)" .)