Thanks Big Dave, I was also stuck on this clue; perhaps you can suggest which way these clues go.
6d. A burden for Kennedy? (3,6)
I have -I-,--R-O
I think the 2nd word is BURRO, small donkey used as a pack animal and that the first word may be pit, but what is the connection with Kennedy?
Also if Burro is correct, the 11 across clue that joins it is A Plant(6). I have E-B-E-.
This is one of eight clues from the new Uxbridge dictionary, so are all puns.
The synopsis from Borders states
If you have ever pondered the meaning of Platypus (to give your cat pigtails), Flemish (rather like snot) or Celtic (a prison for fleas), then this is the book for you. With nearly 600 new definitions from radio's best loved comedy show, I'm Sorry I Haven't A Clue, The New Uxbridge English Dictionary pushes the boundaries of the English language to new side-splitting limits.
I will probably buy a copy of the dictionary for reference in the future