I fear it may just be that "jam" = "traffic" (= "traffic jam"). This would be a pretty weak clue, since the use of "jam" in what passes for the wordplay is pretty much the same as the final definition, but some crossword clues are pretty weak.
The clue can be split into two parts and is based on two different definitions of the word FUDGE.
Newspaper had a box.
Basically a (fudge)-box which can be just called a Fudge is as merriam webster dictionary defines it 'a newspaper space left blank for the insertion of last-minute items.' So this clue hints at FUDGE.
for the sweetmeat
The for is merely ...