"Say it ain't so, ___!" (3)
'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" .)