Say "It ain't so!" (4)
'say it ain't so' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for deny that I've seen before include "Negate" , "Refuse to accept" , "Reject as false" , "Repudiate, disavow" , "Refuse; repudiate" .)
(Other definitions for joe that I've seen before include "Boy's name" , "--- Gargery (Great Expectations)" , "One of spelt out words" , "- Bloggs; - Public" , "-- Bloggs, ordinary man" .)